I'm hoping to grow some hazel nut trees but I can't find how long it takes from flowering to ripe nuts ready for picking.  I know the tree can live in my zone but is my summer long enough for the nuts to get ripe?  The vendor didn't know, and I don't want to invest in root stock if I'm never going to have a nut harvest.  Olive trees grow here but you never get a harvest because winter arrives too early.
Johanna Breijer

Can you give us more information about where you're located? I know that in the US and Canada hazelnuts grow over a very wide range. People grow them in California, the Pacific Northwest, the Midwest, and the Northeast, as well as in many of Canada's provinces

As James asked, more info please.  And more info about where you plan to plant them.

Three thoughts:

If you are planting them on a south-facing slope, it'll be much warmer than flat land, or land that slopes any other direction.  For every five degrees of slope, it's the same effect as living  100 to 125 miles further south.  So on a 20 degree slope, it's the net effect of living 400 - 500 miles further south.  That's significant.  

If you are planting them in a micro-climate where there is a heavy heat-sink nearby (like a block wall, a street or sidewalk, or some other masonry structure like a stucco house that will absorb the heat during the day) that will hold the heat later into the evenings and help your trees significantly.  In particular, if the heat-sink is located on the east side of your tree(s), the hot afternoon sun will warm the wall/house/street,/whatever and significantly raise the ambient temperature.  Most nut threes like the heat in the late summer and early fall months to ripen the nuts.  So even if you live further north, you can bump the micro-climate up a zone or two if there is a masonry heat sink to grab that afternoon solar energy and radiate the warmth into the evening.

I've had good luck growing almonds and pecans, even though we are not supposed to be warm enough for them to thrive here.  But it's all about careful site selection.  The west side of my house gets blazing hot in the afternoons because of all the masonry I have (a paver driveway that runs along the side of the stucco house.  Nut trees serve to throw some shade on that side of the house even as they thrive in the heat of that location.  The problem is the solution.

Finally, with climate change, you might find yourself with far warmer summers than you are used to.  Most people are experiencing this.  It seems like I don't get half the chill hours I used to get in the winter, and August and September are much hotter than they used to be.  If your land isn't warm enough for nut trees today, perhaps they'll be warm enough in about 5 years when the trees are big enough to begin bearing.

Best of luck.

Alot of nurseries list their bloom and harvest time.
But in general it is the same as peaches. Most fruits bloom in April and Ripen in Sept.

We are in zone 8. Our Hazel trees flower in late February and we usually have ripe nuts by late September/early October.

Can you give more climate info or at least location info so that we can figure out your climate/eco-region.
You said olive tree, so I am going to assume you are at least zone 8, but you got no fruits because you don't get enough sunlight or long enough growing season. Based on all of that I am going to go far out on a limb and assume that you are in the PNW in USA or similar climate.

Look for a cultivar that is named after something in your region. e.g. Willamette Hazelnut Tree
https://onegreenworld.com/product/willamette-hazelnut-tree/

Try and find a vendor that actually grows their own trees, even if you have to have them mail the plants to you.

Hazel are like apples or honeyberry, you have to make sure that you get two different cultivars and not just any two different cultivars. You have to make sure that the flower bloom overlap so that vs them blooming weeks apart. And that they are in the right group.

Next is to get a cultivar that the earliest ripening date.

Check out some info from a university agriculture department near you.
https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9073.pdf

See if you can find some wild ones near you and bring seeds/cutting home to use.

Hi
I live in Alberta zone 4.  We have snow at the end of June usually ruining apple blooms, etc.  This year our first snow was the last weekend of August.  The trees can grow here, but I have to know the bloom to ripe nuts, to see if they can produce a harvest in my area.  We had a couple of snow storms in Sept.  So it is a very short growing season.  I asked the company that sells the nut trees for the time of fruit set to harvest and they couldn't tell me.  They want to make a sale.  That is why I asked in this forum.
Johanna

These guys grow in zone 2, so I know you will be okay.
Make the plunge and get quite a few of them.  
https://www.nuttrees.com/edible-nut-trees/other-edible-nut-trees/

I bumped a Hazelnut topic on Permies last year with inquiries, and after some research Hazelnuts actually seem to grow better in the Wild up north rather than south, so one could speculate they are able to produce pretty well with our short seasons :)

This Alberta nursery says 2a, which would be Fort McMurray territory. I'd go with S Bengji's advice and take the plunge.
https://treetime.ca/productsList.php?pcid=114&tagid=8

You could try ordering from GrimoNut, as their parent stock is based off plants grown in the prairies, but are hybridized. They are from Ontario and the site says that the nuts ripen at the end of August, but since the prairies get the most sunshine-hours in all of Canada, it helps us catch up with our shorter season, so I'd say end of August/early September sounds about right. They sold all their hazelnuts before March last year when I was checking the site. I have never dealt with them, so I can't give much insight other than that. (they have good hazelnut credentials though)

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An interesting observation this year: I was repairing fence on some rented pasture and the owners have a u-pick type setup, where they apparently got very few berries because of cold spring weather. But while I was fencing only 1km away from that u-pick, I stopped for 15 minutes to eat wild serviceberries(saskatoons) and every plant was filled. I could be wrong in my hypothesis, but since the wild plants were located half way up a hill, the frost didn't do damage as it couldn't accumulate like it did at the bottom of hill where the u-pick was located. Also, about 300ft down the hill was a swampy area, which I guess helped to moderate the temperature at night.

So as Marco notes, have a good location planned out and there is little reason to worry. There isn't much you can do to mitigate the negative impacts of snow in June though, besides curse :P (I was one of the few who was in the path of a 2km strip of ping-pong sized hail during a storm in July, and that's what I did lol)