the wrinkles don't "heal", it's just that repeated use of Botox retrains the muscle and you don't do the action.
as for every 3 months, depending on how frozen you want to be you can get away with every 6 months with a light touch-up at the 3 month mark. i don't like to be completely frozen, so mine lasts about 4 months. but, again, that's because of my bad habit of raising my eyebrows ALL the time when i'm reading. i work on a computer all day - even co-workers have mentioned to me that i do it. and i can't exactly wear Frownies at the office...
Botox is a LOT cheaper in Canada for some reason; most places in my small city charge $10 a unit. and you often find it reduced to $7.