well if your getting whooped awefully and you just cant take it, the guild leader can push his lil surrender button that follows a vote by the guild, there is set finish time :p its all on how good you play.
I once posted a suggestion thread, a while back, about having two siege war every day. One at 7 PM PST and one at 7 AM PST. It wasn't for any selfish purposes as I wouldn't be able to make the 7 AM one but I thought it'd be nice for those people that are NEVER on at 7 PM PST to get dibs on a siege war.
Then I thought probably the RoM server can't handle so many entries because looking at it now, the system can't even handle the maximum number of guild siege war signups and it's based on first-come first-serve basis.
I think before any advances can be made with siege war, RoM first needs to take SW out of beta except I won't get my hopes up on that since by that time I will have gray if not white hair.