I go until there is reflux at the top of the flask, and then two minutes or so. This ensures that the entire interior has been cooked to 100degC.
Couldn't you just starsan your flask and then boil for even less time?
Personally I boil in a small saucepan for 5 - 10 minutes with the lid on the last few minutes. I sanitize my flask, then i put the hot/boiling wort in the flask, and then put the flask in the ice bath with aluminum foil on the top.