Long enough to give them a chance to comply but not so long that thy forget the instructions
It also depends on what we're doing. Let's say I need them to clean up their room because we're going to be leaving. I might pop in and do the first step, "You need to clean up the toys. We will be leaving in a few minutes." And then I'm off to take a 5 minute shower and get dressed. Pop back in and do step 2, "You need to get everything cleaned up." Then I'm off to do a few more things and finish getting ready and then I come back and offer step 3, "You're having a hard time cleaning up. Can you do it yourself or do you need my help?" If they say they can do it themselves I stand there and watch them get started and then might pop out and do something else and check back to make sure they're cleaning. If they say they need help or I check back to find them playing I stay and, "Here, let me help you," and get them organized and cleaning. but let's say we're getting someone dressed. I will stand with them and step 1, "You need to put your shirt on. We're getting dressed now." Wait a beat. "You need to put on your shirt." "Can you put your shirt on by yourself or do you need my help?" wait a beat "here, let me help you."
does that help?