I got a conservatory built two years ago with pilkington self cleaning glass. I regularly clean two sides but have never touched the other side. All 3 sides are just as clean as each other, and before anyone says...I clean the two sides very well!!

How it works.. (apparently!) There is a coating on the glass, A bird poops on it, Sun breaks it down to a powder and rain washes it away.
Obviously, when I clean the 2 sides I clean it before it has a chance to go through the 'process' but in this country we are never too far away from a good ole rain so the untouched side gets cleaned.
However, frames are getting grubby now and it wouldnt wash away debris like moss from your house roof or leaves etc.