Sessions happen as sessions do, and last as long as the pub will have you! Speaking from experience of last year, the sessions seemed to rotate around the various pubs in Innerleithen so if there's nothing going on in the first pub you come to, either start a session yourself or keep hunting and you're bound to find one somewhere. The pubs are all fairly close together so not too difficult. Owners of expensive guitars (and other instruments) beware the manic dog in one of the pubs (can't remember the name): he likes to jump all over you and scratch your guitar with his claws on the way down! (I'm rehearsing "Dead Dog Scrumpy" just for him!!)