It would partly depend on how many eggs. Some incubators only hold 3 eggs, some hold a few dozen, some hold hundreds.
For just a few, I would open the incubator, candle each egg, then close it up (assuming you can make the room dark enough to candle effectively.)
But if your incubator holds hundreds of eggs, I would take some out and close the incubator, candle those eggs, then put those eggs back and take out another batch to candle. That way you don't have the incubator open for very long stretches of time.
For a few dozen eggs, if you just want a quick peek inside each one, I would probably leave the incubator open the whole time. But if you want to look carefully and mark the air cell and compare with a chart and then look in the egg again, I would take out a few eggs at a time and close the incubator while you are looking at those ones.
As a general thing, I would not worry about keeping the incubator open for up to 10 or 15 minutes, and probably not too concerned up to about 1/2 hour. Likewise I would not worry about having specific eggs out for 10-15 minutes, and probably okay up to about half an hour. Exception, leave the incubator closed during lockdown (last three days), and don't try to candle during that time.