Share 'how much fabric is it ok to impulse buy?'

This might seem a silly question, but please bear with me.... I'm still a novice at this!   When you see a fabric you just love, but you don't have a specific pattern in mind for, how many metres should you buy? Do you do this and if so do you have a sort of rule in your mind for each width fabric or do you just but a certain amount and hope for the best?   Thanks for your  thoughts x