While many fans despise the "Seinfeld" finale, and while even Jerry Seinfeld understands why "The Finale" didn't land with them, it was still a fitting end for a group of characters who, let's be honest, had gotten away with far too much over the years. But the open-endedness of those final moments left fans questioning where Jerry and his pals ended up.
Unfortunately, no answers have been provided in the many years since "The Finale" aired. Even the "Curb Your Enthusiasm" episode "Seinfeld," in which Larry David reunites the cast of the sitcom for a reboot, avoids the subject entirely, though it does feature a new, non-canonical "Seinfeld" scene in which Kramer notes that George can't "make it on the outside" of prison.
With no additional information, it seems that the core quartet of "Seinfeld" most likely served out their entire year of prison time, no doubt instigating just as much shenanigans behind bars as they're wont to do on the streets of Manhattan.