Okay… science answer:

You grow in at least 80% of your hair during puberty. When you have laser hair removal the hairs it kills will never grow again. But you still have up to 20% of your normal thickness that will grow in throughout the rest of your life. So people get touch up sessions every 3-5 years after their big 6 treatments. You need about 6 treatments, one each 2 months for a year, because the hair grows in different stages and it can only kill hair in the active growth stage. The laser works by targeting dark color (the hair follicle) and passing through light color (the skin) the hair and skin have to be very different shades of color for the hair to work. The darker and coarser the hair and lighter the skin, the better results you will have.

Okay… Personal Experience:

I've had 2 out of my 6 treatments so far. It HURTS! It's like being poked with a sewing needle over and over, but it's not as painful as a tattoo, and that's not as painful as childbirth, maybe that can put the pain in perspective for you =) I've had my legs, underarms, and bikini area done. When you get your bikini area done they do ALL of it! Including spreading your butt and vaginal lips apart to laser all the cracks… It's sorta awkward. After the 2 treatments I've had so far I've lost about 60% of leg hair, 40% bikini hair, and 25% underarms hair. It's because the underarms hair is the thinnest and the leg hair is the coarsest. But the legs are the most painful. The more it hurts the better it works.
I spent this much for 6 treatments in each area:
Bikini-$750
Underarms-$750
Legs (toes to bikini)-$2,500

Email me if you have any more questions about it!
References :
Done it all!