There is a 10 day grace period from the bill due date. You can also contact customer service and make payment arrangements if you see it's going to go past the 10 days.
Paying the bill late can also result in a late payment fee on your next bill. All if your bill is due it would make a difference if you are carrying a balance forward for last month or a currently under a payment arrangement.
you do get a grace period and its longer than 2 days, but i would call them and just let sprint know it will be late. they are also good about setting up a payment arrangement if you ever need it; thats one good thing i can say about sprint, i'd had times when i couldnt pay the whole bill and they will work with you most of the time.