It's all about managing expectations. If your boss gives you a task and budgets 5 days to it, then do it within the 5 days. If you know up front that there is not enough time budgeted for it, then raise that. Your manager will like you better for it. If you finish early that's good, get started on your next task or learn something but don't tell anyone. Better yet, if it is a repetitive task figure out how to make that task more efficient. If you are a programmer - write a script, if you are a lawyer/accountant - develop a pre-filled template, etc.
Doing this will set expectations from your boss, manager of what you can and can't achieve in the timeframe given, and whether more resources are needed.
I can't speak for Americans, but Australia has a contract for specific hours - around 37.5 hours in a week is the government standard. To that end we can be upfront with managers/bosses about expectations and resources and usually it is dealt with.
Source: ScrumMaster and developer