How long do I need to wait?

I had an S&T interview at a BB in Jan end, but have been asked to wait. Though I don't have an exploding offer, and almost everyone I know was rejected after being on the waitlisted for max of 3-4 weeks by the firm, I continue to be on the waitlist ("we still have not been able to come to a decision").

Last week, I wrote to the HR telling them about this offer and the fact that I have around 2-3 weeks to decide. They told me that they'll let me know in another week or so.

Offers are for a max. period of 4 weeks, so ideally I should have heard in this week.

What is it that they're taking so looong?
Could you please guide me as to what can be done?

I don't wish to mess up this opportunity...