In most cases in the Pacific Northwest we recommend sealcoating your new asphalt the same year its paved and every 4 to 5 years after. Asphalt sealer will greatly extend the life of your asphalt.
Do not underestimate the value of protecting your asphalt, It can be quite costly to replace. There is not a set cost for sealcoat. If you keep your driveway fairly clean and moss free chances are you will be paying a lot less than a similar driveway that is dirty and covered with moss.
Asphalt sealer is designed to protect your asphalt driveway or parking lot. It protects it from the ultra violet light and radiant heat from the sun as well as relentless rain we experience in the Northwest. Sealcoat also protects your new asphalt from little accidents such as oil spills. It acts in a similar way to paint on wood. Its water based and goes on cold.
Various different ways to sealcoat. Some contractors spray, some squeegee and some broom it on. The objective is to cover all of the existing asphalt with sealer. Once it dries it can be used. This drying process usually takes less than 24 hours.