If you are talking about he brief flush before every use, you do not need to dump any product water (there should be no product formed when flushing...only the outside of the membrane needs to be flushed).
If you are talking about flushing the preservative from a brand new membrane, I would let it run product water to the drain for 24 Hrs. We once wiped out an entire raceway by only flushing a new membrane for a 6 hours (that stuff is TOXIC to corals!). The last membrane we changed, i let it run to the drain for 3 straight days just to be overly cautious.