First let me say, I've never tried living off of just perennials. So what I have to add to the conversation comes from my
experience growing a combination of perennials and annuals as my food source. I personally believe in diversity. I also believe in networking with people in my community, thus trading my excess for items I don't raise myself. This way I can add fish,
beef,
milk, cheese, bread, and a number of veggies & fruits that I don't grow on my own homestead.
How much space? Even if you live in zone 9 and get plenty of rain, it depends upon what you intend to grow. Some crops require a lot more square footage than others.
Trees aren't nearly as productive per square foot as annual crops. My own approach to "how much" was to purchase as much acerage that I could afford, then start a garden & orchard, then keep adding until I was producing
enough. I found that the annuals garden didn't have to be as big as I thought but the orchard space needed to be tripled from my original plans. But since I started adding
permaculture methods to the farm, ncluding
polyculture, now my orchard space could be reduced if I wanted to. This....my current "gardens" are about an acre total or a bit more, and my orchards about another acre. Since I'm planning on adding grain crops, I'm anticipating adding an acre of assorted grains, but I'm looking into incorporating them in with some of the orchard area. (My growing areas are spread out, so it's hard to say if they are an acre exactly without actually using a tape measure to calculate the space.)
What to grow? What you like to eat. While sweet potatoes produce abundant greens, if you won't eat them it makes little sense to grow them. In my own garden, I can grow potatoes extremely easily, but hubby and I don't eat much of them ourselves. But luckily they are a popular trading item. But we do like green beans, tomatoes, broccoli, cabbage and more, so I grow that. Plus on the perennial list, I grow bananas, pineapples, tangerines, and papayas for ourselves. Other fruit trees and fruits, mamaki, moringa, pipinola, various herbs, and mints produce more than we need, thus go mostly for trading.
Live entirely off a perennial garden? If one had enough land, I suppose one could. But it would lack diversity, for sure. Most of what we eat consists of annuals, not perennials. But perennials make an important contribution to our overall diet. Plus, most of my perennials don't produce harvestable food year around.
Very little daily maintenance? Gee, every new gardener who attends my teaching sessions asks that within the first month. They want to grow lots of food with little or no work. Personally, I don't see that happening. Farming and/or growing all of one's food takes work. Now, if one wishes to live off of just acorns and sweet potato greens, then the work will be minimal, but there is still some work involved. Having said this, I find on my own farm that the more acreage I have in perennial polyculture, the less work I put into it per square foot. The growing is not as intensive, thus doesn't need as much intensive attention. But it also means that I need to spend more time walking, hauling
compost and mulch around, and that sort of thing. Thus I've traded intensive physical digging in a tight garden space for lots of walking and hauling across acres.
Bio-intensive is labor intensive. Not that you'd spend all day working, but it isn't what I'd term low maintenance. And besides, it's a system designed for annuals.
For what it's worth, I normally work 17 hours a week on what I call "playing with my food". These are jobs that I take great pleasure in doing and includes tending livestock,
greenhouse work, garden work, irrigating where needed, harvesting. That doesn't include my time spent processing compost, mowing, creating & working on hugelkuktur pits and other projects, fixing and maintaining equipment, foraging, trading my excess, processing my harvest. These latter are my afternoon jobs which get mixed in with house construction, general farm maintenance, shopping, off farm appointments, meetings,
volunteer work, and the myriad of other things I do.