I have subscribed to UWorld but haven't activated it yet. I would like to wait a while before I do. Would it expire if I don't activate it?

don't think so. The timer starts to run once you activate it, and it works separately for uworld qbanks and each self-assessment form. For example, if you bought uworld qbank with 2 forms (360 day plan) on 10/28/2015, but you activate the question bank only on 11/2/15, then the qbanks will expire on 10/27/2016. The other two self-assessment forms can be activated separately, and you can assess each form for 2 week, starting from the time you activate it. Hope it helps!

Thanks ADBRuby!