Woah. I'm going to have to go through all of my Pink Floyd Cds for this one. Probably shortly after Syd Barrett went loopy. Their style completely changed after that, so that's when I assume Gilmour started kicking ass.
[EDIT]I was half right. There was a stupid little solo on the Floyd's first album. Not "proper" though. I assumed then that it would be on A Saucerful of Secrets, but that wasn't the case either. Gilmour did kick ass, but there were no real guitar solos. The first "proper" one was on Echoes, and even that wasn't your everyday solo. Some people may have a difference of opinion on what a "proper solo" is, so some might say that it started earlier. I'm pretty sure that meddle was the first, though.