When you get to the ferry-port in Ibiza, there are usually two or three windows open, with the ferry time (and company) clearly marked on the TV screens above.
If you get a day return, then you must come back via the same company you went out with, so grab a time-table from the window you book your ticket from.
Just looking at Balaera, they have announced their early summer schedule, looks like the last ferry back is aroud 9pm.