I planted a couple bare root black locusts about two years ago that I want to coppice.  They are about 8 feet high now, and their trunks are about a couple inches in diameter.  How long do I need to wait before coppicing?

From what (little) I have read, and with no hands-on experience yet (ie, grains of salt should be taken here): you should be able to coppice this year, ideally once the tree is dormant and average daily temperatures at freezing if possible. Cutting it at the diameter you need it is the biggest factor, I've seen some cut at just over 1" for tool handles, and I would guess 4-6" diameter for RMH feeding would let you drop one log in after it's heated up and you are good to go.

black locust from seed/seedlings is one of my goals when I get property, hopefully a couple years before I start building so I have fire wood ready to coppice. Getting logs 10-12" thick for building a house might take a bit longer though.

Good luck, post some pictures for us "armchair foresters" to look at!

I'm mostly growing them for the nitrogen, next to some fruit trees.  I live in California, so average daily temperatures are always above freezing.

I would wait until your coolest months then, January or February, to cut. Just minimizes wasted energy being pushed up into what you're about to cut so the roots can instead it into new shoots.

There is an Ontario fruit orchard on YouTube that converted to a permaculture model that you can check out, I think they alternate apple varieties, a nitrogen fixer, and pear tree varieties. I think it's honey locust but I might be wrong. They also plant shrubs and perennial herbs and veggies in between and got enough varieties to allow bees to harvest all season long and placed many bird houses to encourage birds to hang out and eat bugs.

I mention that because as you coppice the black locust the new shoots will start to creep out and not stay in the same spot where you want them to be. So perhaps a honey locust that you don't prune or maybe nitrogen fixing perennials might fit the bill and be less hassle? I like the idea of having a nitrogen fixer surrounded by 2-3 various fruit trees that flower at different times so the bees benefit, and having various shrubs and perennial around the trunks to fill it all in.

geoff lawton suggested in his recent PDC that coppicing is best done at the end of the growing season (fall) when precipitation exceeds evaporation, when leaves are at their most over-mature but the stalks of the plants still have enough sap to heal from the wound before winter dormancy, but also that some of the sap has already returned to the roots for winter. 

In the spring, the stored sap will rise and become fresh shoots for future coppicing.  Coppicing creates a compost corridor as the roots related to the coppiced branches will, over time, rot in place, and for your needs will release the nitrogen that is related to the roots which correspond to the particular coppiced piece. 

If you snip up the coppiced branches of a nitrogen fixer into smaller bits, they will provide more readily available nitrogen as chop and drop mulch.  It seems that you have mature enough stock to begin just trimming some smaller branches and feeding them to your fruit trees on the surface, while potentially directly benefiting the fruit trees with root based nitrogen transfer, if they soil communities have merged. 

Regardless, if your nitrogen fixers are close enough to your fruit tress the soil communities will eventually merge, nitrogen will be exchanged, and coppicing is not as necessary... though you may desire to do so anyway to increase nitrogen for your fruit trees, to mulch them, to provide other trace nutrients/minerals, to give them more light.