Officially, I think the band will continue to exist as long as John Petrucci can play a guitar. As far as being a an active, full-time band, though, I'm thinking about 15 more years, give or take. At their current rate, that will be right around the time they release their 20th album, which will also be around the 40th anniversary of WDADU, and the 45th anniversary of the band. By then, Petrucci and Myung will be in their early 60's, LaBrie and Mangini will be in their late 60's, and Rudess will be into his 70's. I think around that time we will see DT making statements similar to what Rush are saying right now, about cutting down on the band's touring, recording and the like. I doubt there will be an actual break up of the band until Petrucci can no longer play music (which for him will probably be 'til the day he dies), but I think once they get to that 20th album/40th WDADU anniversary/45th band anniversary in 2029-2030, they will scale back their activities as Dream Theater drastically.