When I start my truck, the headlights come on automatically. Because of this, they get alot of wear. Are they getting dimmer with wear or not? How often should I change the headlights - only when they go out?
Those are DRL's (Daytime Running Lights) I would get use to them as I suspect they will be required in the future.
Most DRL's run the lamps are about 75% of full brightness. This means they will get nearly the same life as if they never came on. The total cost is minimal.
Studies indicate that they do reduce accidents, but not be a great deal. I for one, want mine on unless I turn them off. (My car has that option, but I leave them on.) I don't know what might be involved in turning yours off, it varies from car to car and I have heard of some mechanics refusing to deactivate them for fear of problems if there is an accident.
I don't have the data, but there is some energy consumed and all else being equal they will reduce mileage, but not enough to worry about.
There are some very valid reasons for having the ability to turn them off. I would hope no mechanic is refusing to turn them off unless there really is a law requiring them in your area.
Good Luck
For the most part the bulbs really only have 2 states - they either work and make light or they don’t. They don’t really lose brightness from “wear.”
The lenses and wiring are a different story but most of those issues are about age, not frequency of use.
Lenses can be cleaned up quite nicely with something like Maguiar’s PlastX. The power supply can be checked with a voltmeter.