How long are you willing to wait to read an update before you give up on a story?
If the story's good, I'll wait for it, period. I don't really have a set time limit.
If the author tends to take a long time between updates, do you find you have to go back and reread the previous chapters before reading the new one?
Yes - if it's longer than a few weeks, I usually have to refresh my memory.
If the author takes too much time to update, do you find yourself less excited to read when it is finally updated or hesitant to read something else by that author?
Usually, yes. There are some stories that I really like, but because so much time goes by between updates (years, in some cases), it takes so much effort to go back and reread and refresh my memory that I have decided to just wait until they are finished before reading any more. I'd rather go back and reread them all in one shot than keep reading a chapter every few months or years and then forgetting what happens.