@denise1Your top up date is 30 days from when you had purchased your plan.
^^^ What (s)he said.
However, since you mentioned "add-on" in the thread title without further details to figure out what situation you are in...
All add-ons except the US roaming add-ons (which are valid for 10days) are yours until used up, whether that's in two days or two years (or until you choose to close your account, of course). The remaining balance of minutes or data will carry over to the next payment cycle.
I believe if yoy call 611 it will tell you renewal date and time