Well that's a bit tricky. there are no set numbers. You'll have to play a bit. If you see the photons there but they look like dots or circles, first decide if you have enough. If you do, fine. If not add more--realize that when using global illumination it is quite regular to have LOTS. Then increase radius until the circles start to overlap and blend; the splotchiness should start to go away. You can and should increase accuracy as you go as well, but that will only solve how accurate a given area's sampling will be.