I typically assume a very conservative 20-foot radius around a ceiling-mounted access point, give-or-take.
Some general placement rules:
Access points should be centrally located to the areas they are serving.
If you have a cluster of bedrooms off of a common hallway, then mount the access point in the hallway, so it is approximately equidistant from the center of each bedroom. Same for the living room/dining room/kitchen.
Don't mount access points along exterior walls, unless you want half of your coverage radius outdoors instead of indoors. Likewise, don't place them in exterior corners of your home, unless you want 75% of your coverage radius outdoors. Keep them more towards the interior of your home, to maximize the indoor coverage radius.