So, I got an itch to do this and went and checked properly, tell me if I've done something wrong and I will update the list.
This is how much the band members spent time together in the band, let's cut the guys a break and let's count 1985 as the starting year of the band, since all those anniversaries that followed were based on 1985:
John Petrucci and John Myung = 33 years
John Petrucci, John Myung and James LaBrie = 27 years
Mike Portnoy, John Petrucci and John Myung = 25 years
Jordan Rudess and John Petrucci, John Myung and James LaBrie = 19 years
Mike Portnoy and James LaBrie = 19 years
Mike Portnoy and Jordan Rudess = 11 years
Kevin Moore and John Petrucci, John Myung and Mike Portnoy = 9 years
Mike Mangini and John Petrucci, John Myung, James LaBrie and Jordan Rudess = 7 years
Derek Sherinian and John Petrucci, John Myung, James LaBrie and Mike Portnoy = 4 years
Kevin Moore and James LaBrie = 3 years
Charlie Dominici and John Petrucci, John Myung, Kevin Moore and Mike Portnoy = 2 years
Chris Collins and John Petrucci, John Myung, Kevin Moore and Mike Portnoy = 1 year
Should the band be active in 2024, Jordan Rudess will have spent the same time in the band with Petrucci, Myung and LaBrie that Mike Portnoy spent with original founding members John Petrucci and John Myung. If we count the active years, from 1989 onwards, it's already a tie.