As regards how long universities will wait before you give them a yes or no answer, that depends on a lots of different factors. if the position you are applying for is a studentship on a very important project, they are likely to want an answer quite quickly if they offer it to you. There might not be much interest in some studentships, in which case you will probably have a lot of time to think about it. I was offered a PhD position a number of months ago and still havent given them an answer. I could probably contact them in a few months time to say yes and it would still be available.
I hope this helps