1 hr to 1 hr 30 typically. Just an fyi to anyone that is questioning the data. I have three sleep trackers. Oura, eight sleep and apple watch. Oura always shows the highest deep sleep Eight sleep and apple watch are very close numbers but occasionally apple watch will show slightly lower deep sleep numbers.
I would say if apple watch is showing ~ 1 hr you are getting enough deep sleep. Any lower is probably a little on the low side.
Doesn’t work half the time so i stopped trying.