It depends, if you are playing with a band, you would want about a minute - 2 minutes. Now this is a cutout solo. If the band continues playing during your solo I would say about 45 seconds at the maximum.
102 hours and 49 minutes, this was accomplished by Allister Brown (UK).
NOTE: This record was set in 2008 and therefore might have been broken. (I don't have a recent guiness book. :P)