How long do I have to make a payment on my ATT PrePaid phone bill?

I recently made a plan change to my bill and apparently the bill will automatically kick the new plan into place kicking the old one out and which now a balance is due and I have no service since it doesn't just kick the plan into place on the next billing cycle. So how long do I have to make a payment on my ATT Prepaid bill before they try and cancel my service and I loose my number??? I plan on making a payment within a few days. Also plan on having it paid with credit/debit card. They don't save your card information automatically do they? because I don't want that. Thanks any information would be useful and helpful to me.