15 is the minimum I think. Depends what you are going to be doing with the final animation. I do mine at 25 FPS because they are intended for PAL television systems. If it is for NTSC television systems then 30 FPS. For the web 15 should be OK, but I have never done it that low, so I can't be sure. At fifteen frames a second I would guess about 45 frames should look OK for a full revolution, but do a test render in cartoon mode or something and see what the movement looks like.