"Great questions. I raise trees from seed, too, and find much pleasure in it.
I don't know what species maple you have, from what you have told me, but aside from smaller species such as Japanese maple, most grow large. Anything with a mature height of 25 feet or more should be at least 15 feet from your house.
You can transplant your maples into the garden anytime the ground isn't frozen. Just be sure to keep them watered, which you've obviously been doing while they've been in pots.
If you want to keep them in pots for awhile, you will have to jump up the size from time to time. A 2-1/2-inch pot is fine for now, but by next spring you'll want to move up to 1 gallon. A good time to make the move is when plants are dormant. Gently take them out of the pot and look at the roots. If they're thick, gently slice into the rootball along the sides and at the base; this will keep them from spiraling around and eventually becoming rootbound. Then repot into the larger container using potting mix beefed up with compost. (I find that potting mix by itself tends to dry out too quickly.)
One caveat: if you have silver maple (Acer saccharinum), keep it far from the house and make sure to prune regularly so you have ONE UPRIGHT STEM. These trees are notorious for forking into multiple trunks which will tear apart in storms and cause much grief. Keep them with one single stem and remove branches that cause narrow V-shaped crotches and you'll be okay.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine"