I'm not exactly sure when I'll stop playing, but I know it will be a long while after 1.1 is released. Even when I've gotten a little bored of the main game alone, I'll always be on the lookout for things like decent characters, large-scale mods, and more.
While the release of 1.1 will cause a huge amount of n00bs to join the boards, the fact that SRB2 1.1 will have a lot of content will probably attract a lot of non-n00bs, and maybe some very intelligent users that will make some pretty big mods for SRB2. I wouldn't be surprised if SRB2 1.1 was in the spotlight of the whole internet for the first few weeks that its released.