I think that they left for a tour something like the day after Patrick & Joe graduated from High School, and at the same time, Patrick essentially moved out from his Mum's house (and lived out of the van)...so I would say that after they got back from that tour, the guys probably got the apartment then. And I think this would have happened in 2001, or 2002 at the latest.

Apologies for no links to interviews, this is what I've pieced together and off the top of head (which is probably pointy). Heh.