The fact that dozens of comments think 90 days is too long for a service to be invite only shows we are really in a instant gratification news bubble here. 90 days! People act like we are talking about Chinese Democracy or Duke Nukem Forever here (not to mention that for about 80 of those days getting in was merely a five minute search process for anyone who really cared). The majority of comments here are about too…
Just FWIW, I used Google+ for 2 weeks or so. Then I got bored and went back to Facebook. I think 90 days is certainly enough time to lose any early momentum you had.
Except for going back to Facebook. :) I got bored with that after about 2 weeks, too.