The top coating of sealant on asphalt protects it from water intrusion, but that layer isn't invincible. Age and damage to the layer can nullify its protective qualities. However, it's kind of hard to tell -- actually, nearly impossible -- if the sealant has been breached unless the asphalt has suffered visible damage. The thing is, you want to prevent that damage to begin with. So, here are some times when you might want to consider getting the asphalt resealed:
The Age of the Current Sealant
Sealant doesn't have to be damaged for it to be useless -- it can wear away over time, too. You'll find numbers vary, but resealing anywhere from every year to every two to three years is common. If it's been several years since you last sealed the asphalt, then it's definitely time to reseal.
The Types of Things That Hit the Sealant
Sealant is meant to withstand some constant pressure, like the weight of your car in a driveway or the trail of bicycle or rollerskate wheels. However, if something hits the sealant very hard, such as very large hail, that can cause cracks in the sealant layer that don't necessarily lead to visible cracks in the asphalt's surface.
After large hailstorms, if someone's been fooling around doing skateboard tricks in your driveway, or if large tree branches have fallen onto the asphalt after a storm, you may want to consider resealing. If the potential damage is in a small area, such as in that tree branch example, you may just want to reseal in that general spot; there's no sense resealing areas much farther away. However, for mass damage like in a hailstorm, it may make sense to reseal the whole expanse of asphalt after repairing any divots left by the hail.
Don't Assume Sealant Doesn't Help
You might be thinking at this point that sealant isn't really helpful if you have to keep resealing. But it does help. Basic sealant -- not just the blackish stuff that makes your driveway, for example, look new, but actual water-resistant sealant -- prevents water from seeping into the asphalt, which is already rather porous. In cold weather, that water can freeze and expand, which cracks the asphalt. If you've got the area sealed properly, water isn't going to get in.
If you're truly unsure about whether you need more sealant, you can always have the asphalt inspected for signs of damage. A trained paving company like AAA Paving and Sealing can spot issues that you might overlook yourself.