According to the Memory Alpha page on shore leave, Starfleet (both Earth and Federation) receive occasional shore leave. One thing the page doesn't mention though is the frequency of shore leave. I'd like to know how frequently shore leave is granted for example in a formula i.e. after 'x' months of duty officers receive 'y' weeks of leave. All I could find from the aforementioned Memory Alpha page was:
by the time of the Ba'ku incident, Picard was owed a staggering amount of shore leave: 318 days, to be exact
I imagine some calculations could be done based on the above number, but taking into account that Picard did have some terms of shore leave on Risa (Captain's Holiday) and Earth (Family), we don't know prior to TNG when Picard had last had shore leave!
I would prefer some sort of evidence which explains the frequency of shore leave (or for that matter if it is granted purely based on how well the crew is coping at any one time, although I doubt that would be the sole reason, as the number of days Picard was owed suggests a more precise system).