Hi, welcome,
I would say from watching Gold Rush, they might do an early clean out if they suspect a problem with a sluice box. Maybe to see if there's Gold or no Gold. Or too much water or sluice angles and all that. Otherwise if it's set up right, they go all day. I think you have to test pan the areas first. I've done really dumb things like sluice all day in a hole with no Gold, that was a waste of time. So now I run 2 buckets in a sluice, and if there's no Gold, I move on. If we spend all weekend sluicing, I want it to be down where the red gravels are packed under a big rock.