the usual mantle life is between 2 minutes to 2 years 
it depends on so much reasons, like quality of the mantle, skills of the user, size of the mantle ...
i have an mantle in my 250 HK in use for over 2 years now, and the lamp is my main camp light equipment in my landcrusier...
There are many ways to kill the mantle, e.g. hard strokes like bumping the fist on the Table can kill it.
Hi Dutch,
not much to add here, rather to sum up:
- you won't have to change a mantle "on precaution" / because it's not bright enough.
- you should immediately replace a physically damaged mantle, it will cause further damage to your glass / lamp will not run optimal
- if it's all black due to being carbonized (coming from a failure to start "cleanly"), it depends: if any errors of the lamp are corrected, and the Petromax is running optimal, you can try to burn this mantle free. This may work sometimes, sometimes not -> another reason to change it.
Just as Martin said: It varies extremely. I've had mantles that were damaged during the first run of a tank full of full, others stayed for weeks or even months, with daily usage, some even despite being transported every day, and with no noticeable "fading" of brightness.
One final hint: store unused mantles in a dry place, and do so with your Petromax as well. "Moisture in the sock", be it unused or when in the lamp, in my experience leads to quicker damage.
