How often does a northbound train (that would go to Grand Central) stop at Union Square Station in NYC @ ni?

I've tried to parse through subway maps and schedules for the 4, 5, and 6 but maybe it'd be better if someone from NYC or with experience with the system could answer my question. How often does a northbound train (that would stop at Grand Central) come to Union Square station at night (particularly between 11-12?) I would like to catch the Metro North at 12 after getting out of a concert at Webster Hall that will likely end at 11:30