We know that microfilm lasts at least 100 years. That is a handy piece of information for archivists, but most other people don't care. How do we know that microfilm lasts that long? Because we started making microfilm about 100 years ago. How long can a forum last? We have to wait to find out.
I stay only as long as it is interesting. After a few months, I had to shift my focus. One can only stare at the boob tube so long waiting to be entertained. Now, I will stay interested as long as I feel I have something valuable to contribute.