Have to be boring and agree it depends on what you're doing! And also what you're doing it for - memories, good keepers, a fantastic collection, or to sell.
Wildlife photos, a few hours and lots of attempts for a good few photos isn't bad - unfortunately the interesting wildlife doesn't pose so well!
When I was at the zoos... 80% were acceptable, maybe half were good shots - including test shots for settings. That being personal work.
I'll expect when initially doing pet portraits around 25% to be saleable - simply due to looking away, badly timed blinking etc.
Oh and taking photos of my brother, 1 in 100 being good would suit, he has a sixth sense for when my camera is near him!