I totally agree with adam green, the spectral mask is such that adjacent channels DO cause performance impacts when AP's in the same area are close together. Seperation (horizontal and Vertical) are best, but that is not always a likely possibility, the solution to this is to use non- adjacent channels, like channel 1 on the first AP, and Channel 11 on the second, this gives me about 20Mhz of spectral seperation (22 in the 2.4 Ghz range) between channels 1, and 11.
If you can't seperate by spectrum, distance is then needed. It really depends on the EIRP values and the IRL received. That is getting more complex, than most should tackle without having an RF Engineering background. As a general rule I would say no less than 10ft. Unless you are going into Picocell mode.