I have several very good friends, one I e-mail constantly but we've only been good friends for 4 years.
The friend I hang out with the most has been my friend since our boys were in preschool, about 13 years ago.
Another friend I have known since 1978. We met when I lived out in Colorado. We still write to each other but haven't seen each other in years.
I still see my HS best friend now and then when she comes "home" to visit her Mother.