They do last.. Sealants are great and worth the initial expense when applied. Each dentist has his own thought on who and how many teeth get sealed. For my patients i usually only do molars unless a person does appear cavity pron or has deep pits and fissures on their premolars. Some dentist seal all posterior teeth regardless. Anterior teeth are very rarely sealed.
Yes and no. Sealants can only be used on the grooves of the back teeth. That being said, the sealants will last a long time, probably not your whole life. However, they can be re-applied if they come off. There are no sealants for the front teeth.
Highly Unlikely. It would be highly unlikely for you to have sealants on all your teeth for life.