How long can you drive your car with an expired decal on your tag?

Depends on the State ... and the cop

Decals should be changed before the last day it is supposed to expire. After that date it is illegal and you could be ticketed.

In my state, the cops usually start looking for expired tags mid-month and start passing out tickets. However, I have seen some state laws that give you 1 month leeway to get renewed decals.