SD/HD firearms have a round chambered 99% of the time - whether in the safes, in lock boxes, in a vehicle, or on my person.
The only exceptions:
1. Changing ammo, or checking function.
2. Actively being shot at the range.
3. Cleaning.
Time? Sometimes days. Other times months.
One of my HD firearms has had the same cartridge chambered for four years, with the exception of one trip to the 'range'. I occasionally drop the mag, eject the round, cycle the action a few times, function check the piece, and then reload it exactly as before (ease the slide down and push it so the extractor locks over that round without bullet set-back). But it has been in pretty much the same condition for quite some time.
Leaving a round chambered risks corrosion from contamination.
Constantly re-chambering a round risks bullet set-back or crimp-jump.
Pick your poison.