Start flushing now. From what I've heard, many companies mandate that you must take the UA within 2 business days of accepting the job offer. That's what happened to me.
Stall accepting as long as you can, and I think most here will probably tell you that subbing is your best option at this point (you didn't mention how much you smoke, age, weight, metabolism, etc though, so it's hard to tell from the above).

Good luck