Recheck it before using it to navigate, as The Rookie said. That includes before using an ADF and before starting your approach, even VFR. It helps you to find the correct runway at an unfamiliar airport, or to find the runway at all if you descend through an area of poor visibility.
Don't forget that the compass is inaccurate during turns and during and airspeed and pitch changes, so be sure it's stablized before resetting the DG.
I too was taught "15 minutes," but the time the DG takes to drift noticeably depends on which way you're going, what exercises you are doing, and on the condition of the gyros and suction pump in your aircraft.