How old is he?
With adults, the 20 minute rule is usually good.

However, if he's still a baby, being out for 20 stressful minutes can put them off feeding, and really won't get you far.
With babies, I find 5 minutes is all it takes, each night (not on or after feedings) and if they happen to strike, I add another 5 minutes. The maximum is 10 minutes for me. It's worked just fine, they still eat, and after a month or so they seem to relax and calm down...until you show them to someone else.