Margot, it depends on whether you've knocked the hook against a stone or some other hard object that will blunt it. And, of course, how long you've been using it.
The wisest thing to do is check it every time you bait up and at the first sign of it losing its edge either touch it up with a fine stone or diamond file, or change the hook.
Modern hooks just don't need changing too often if they haven't been put under any stress. However, if you've caught a number of hard-fighting fish on one, or been snagged, or anything has happened that could have tried to straighten it and therefore weakened the material, then change it to be on the safe side.
It's not worth taking a chance for the sake of a hook.