it depends on how and who is going to quilt it...
some long arm quilters request 4"-5" all the way around---meaning if your quilt top measures 80x90 the backing would be good at 90x100
if quilting with a domestic machine you might not need as much (not attaching to the leaders which the quilt is rolled up with on a long-arm frame)
if hand quilting it-that would require something different too-
if you are sending it out to a long-armer you need to ask that person- different ones have different (requirements)
everyone is different.
i ask for an extra 5" all the way around but when there is less it can *usually* be dealt with. more is better than less though when long-arm quilting