Most homebrewers boil off far too much simply due to small volumes and (relatively) large burners. Just how much boiloff is necessary is system-dependent (elevation, kettle geometry, calandria vs. natural convection vs. recirculation, etc.) but 5-10% is definitely all that's needed to volatilize DMS. FWIW, I boil off ~12% on my 6 gal batches because that's the lowest I can throttle my burner while still getting a clean flame.
A longer or more vigorous boil can be desirable in certain situations where the Maillard products are essential to the beer (doppelbock e.g.), but boiling off 2.5 gal in a 5-10 gal batch is probably just wasting fuel.