In the beginning, they will have the servers up for short periods, then down for maintenance, patching, updates, etc. As time goes on, the servers will be up for more and more time between down-times, because the client will become more stable and less will be needed in the realm of fixes and updates.
By the time they get to beta weekends, the primary testers (the ones who have been there from the beginning) will be able to play most of the time and the servers will blow up with TTD (ten-thousand-dweebs) on the weekends.
This is the standard for MMO's. I've seen it through at least a dozen titles.
So when it starts, we'll be out more than we're in but as time goes we'll get to play more and more...Also, it's going to be quite a while before the game releases so unless they move up the release date, we won't get in to beta weekends for a few months.