I have had the same light on mine for almost 10 years now, and have never had to replace the bulb. Turns out it is not an actual "Mag-Lite" but a "Brinkman" instead. It is one of the 6" or so length doodads that takes 2 AA batteries. It is bright enough to light up an otherwise dark room and highlight the aim point, but not as bright as some of the newer LED flashlights.
Also, part of the reason it may have lasted this long is that I have only put maybe 50 rounds through that gun since I have had it. The pistol grip, while being all rambo and everything, is not ergonomically effective ... imagine a hammer smacking you in the soft spot between your thumb and index finger ... the pistol grip forces this area to take the entire brunt of the shock, which can be substantial when you put some 3" 00 Magnum loads thru it, so I am not fond of shooting it other than the annual aim test and cycle.
The purpose-made firearm lights ( Stinger, etc) are pretty expensive- I have one that cost as much as my shotgun...