There are two types.
Paper type. Not reusable. Throw it away.
Reuseable cotton/plastic type.
2. Many people have used reusable type for years, that is how k&n filters makes money. They don't filter as well as paper type, but it is a compromise when you want reusable and want to make power(less restriction/less filtering). Don't listen to your mechanic and find a new one. Ask him if he replaces the crush washer when changing the oil drain plug out.
3. Read the manual or just do it every year or two. It depends on where you drive also. I cleaned my afe dry filter at 10k miles and it was very clean. Look it up on google on how to clean it. I just hosed mine off from the opposite direction of the air inlet until the water was clear and let it dry overnight.