well it could be just about anything , but usually if a light bulb has too short of a life <7 minutes is way too short> it is getting too much voltage. whether the source<lighting coil/alternator> is putting out too much or you have some type of wierd ignition problem back feeding thru the chassis. Unless it is a mechanical issue< breaking or lots of vibration/ shock>. Take regular house bulb , put into a drop light and drop it and you will know what I mean by that.
would you happen to have a volt meter?