I think the real question is about reinstatement. It must be pretty easy if we go by Worf's example. Clearly he had to resign if he took up on a wartime Klingon vessel. It would seem all it takes to be reinstated is for your former commanding officer to take you back
Seeing Wes back in uniform for Riker's wedding would also indicate a rather loose open door policy
Well in TMP we learn that Spock left Starfleet to return to Vulcan and was listed as "Starfleet - Innactive". He is imediately reactivated by Kirk once he returns to Enterprise. Maybe you never REALLY leave Starfleet. the movie certainly indicates reinstatement is a simple matter of logging it.