How often did sub sailors get home during WW2?

Not very often. The needs of the war came first. Sailors might have had a chance for leave and travel home when reassigned to another boat, particularly if it was to new construction. They would also get some leave during shipyard overhauls. During an overhaul, captains would try to give half the crew off at a time, usually starting with the married men whose wives were not nearby. Then the rest of the crew would get their chance to go home for a little while.

All of this was true of the rest of the Navy as well.