silverelise they cost about 25 quid a pop and last between 10 - and 20 hours depending what you do with them.
The pitot heaters last a hella of alot longer but its the same thing its all about cost.
Its not uncommon in some schools to have a set of working ones which get swapped back in for a 50 hour then when the aircraft gets back to swap blown ones back in.
In a training environment they get heat cycled every 40 mins you sometimes can be lucky if they last a days flying.
Must admit I never used to turn them on either but if if someone wanted them on I didn't bring the subject up. The school owner if he spotted they were on most certainly would bring the subject up. Most of the time unless there were night ratings in the offing they would be duds anyway.
And on the subject of Nav lights I know you don't have to but they really do help if someone is trying to avoid hitting you if you have them on. Nav lights and strobes work for me day and night.
There is alot of theory out there that it makes you more visible but in reality the landing light has a very small arc in front of the aircraft which it is doing any good and also it is pointing in the direction which you have a very good view of anyway.
Even at night you need to have them working but that doesn't mean you have to use them. I got MOR'd by a tower for landing with them off on a dark stormy night with a cloud base of 001 bkn and RVR of 700m and thick snow falling. If you had them on you could barely see 5m past the nose and the glare was effecting my vision. With them turned off it was fine picked up the approach lights at 300ft and had good visual references for the flare. The company never even heard from the CAA asking to justify my actions so I presume it got to the pilot side of things and was stopped there.
It can be a very emotive subject and to be honest I expect a bit of flak for what I have written. There is no requirement to have them turned on for day VFR in fact quite a few aircraft don't even have them fitted. And the benefits in the grand scope of things are relatively small compared to maintaining a good look out around the full arc that you can see. A fancy high viz paint job on your aircraft is more of a factor than having a working head light.