I can almost gurantee in the year 1999. My theory is based on things that Jerry said on Leno.
1."Lets do a 10th session." Jerry never ruled it out after he said it 2."I just need a break for a while." 3. The Fab Four got a year. If Seinfeld wasn't going to return then they would have killed them or sent them away for like twenty years. 4.Jerry told Leno "I still have the set." If he wanted it for sentimental reasons he would have kept the Superman statue.
Anyway I think Jerry just wants to tour a little get back to the basics. He most likely taking a year or so off to work on new scripts because he was disappointed with the last two seasons and want a chance to make it a better show before it got canceled. People will want to see a reunion but if it got canceled then it would be like oh that's so sad there giving it one last chance.