Hehe.. I think it varies. You can always browse through the posts and get a general idea. In addition, before making a post, double check to see if it's been done before with the search feature (especially if you haven't been around for a few days).

I think that there are about 40-60 new posts a day (not including any responses, etc.).

Anyway u cant be ahead of things all the time.
No but even if you get a general idea of what is being talked about is helpful. I remember being an admin spending an hour each morning just going through AO and other security sites. I didn't read everything but I did get some ideas of what was going on. In addition, I'm subscribed on 4-5 of the bugtraq lists to see what new vulnerabilities and other things are being discussed. Do I read them all? No but I at least know what issues have appeared. I can look them up afterwards if I need to.

It's better than not knowing.