I’ve fallen a couple times, but no serious damage is visible on the helmet except for a couple divots here and there.
Should I consistently be replacing my helmet after a certain amount of time? After every relatively serious fall?
Helmets should be replaced roughly every 5 years and after any crash where your head makes contact with the ground.
Helmets will crumble, compress or otherwise deform in sometimes hard to see ways when you hit the deck. The structural integrity of the thing will be massively diminished after even a relatively tame fall. This is one area where you don't want to mess around much.
When to replace a bicycle helmet:
The general rule is every 3-4 years or after a crash. Some people say that excessive sweat or UV exposure can also degrade your helmet, but no one has proven that so far.
Why are you using a helmet?
If you're using it because you're doing something which involves a decent probability of you falling and hitting your head, then you should probably replace it after every serious incident. Some companies (e.g. Giro) will give you a discount if you trade in a damaged lid, so that they can learn from its demise.
If you're commuting or doing something less dangerous, but which offers some scenarios where you think a lid will assist, then you might be less concerned about replacing after every ding. But the helmet is designed to work when pristene; as soon as there is damage, any damage, it won't act in the way that is was designed to, so is going to offer you a sub-optimal service.
But if you're just using a helmet because you think you should, although remain generally unconvinced (peer pressure, race rules, certain state/national legal obligations, enhanced air flow around the scalp), then don't worry about it ...
Bicycle helmets are designed to absorb impact by cracking or crumbling. The insides of the helmet may crack with no damage visible on the outside. I'd replace the helmet as soon as you can.
If there's visible damage to your helmet, like a crack, then you should definitely replace it. Dents, maybe, it depends how bad they are. Better safe than sorry right?
Inspect the helmet for damage inside and out. If you see any cracks, then it's time to replace it. If not, you're fine. You don't want to risk your cranium when you could just buy another helmet.
It depends what you use it for. The primary advantage I have from my helmet is not hitting the branches with my head, when riding in forest. For that I don't need a new one each 2 years. I even heard that this is actually what bike helmets were made for in the first place.