[chris vickery]

Jes
I never knew much about fit testing until I actually went for one. We were assigned to work at Koch fertilizer in Brandon MB. As Rob L will attest, this place is basically a death trap where everything can and will kill you. They are makers of very nasty items like Anhydrous ammonia to name but one...
When I went through a fit test the representative at the safety supply in Toronto actually puts you through a video presentation, followed by a knowledge test. They try different masks on you to determine the best fit. The mask is hooked up to a computer and air system which checks various items. You are exposed to a smoke test. Once all this is done you are basically ready to don and use respiratory protection.
The one issue that surprised me was actually how snug a respirator should be while worn. Tight actually. I think too often the home mechanic thinks he is doing good but is actually doing very little. A bad seal is the same as none.
And forget those cheapie P95 white dust masks- they are only good for nuisance dusts, eg saw dust, dirt etc.
As far as life expectancy of filters, it all depends on use. My 3M half mask uses the charcoal cartridges as well as snap on pre-filters. The pre-filters grab the large particulate matter to keep the charcoal from getting clogged up quickly. The charcoal provides the chemical vapor filtration. Correct filter selection is vital- the wrong filter can kill.
The worst thing for the life of any mask is improper or no maintenance. After each use, the filters should be removed. The mask itself should be washed in mild detergent and allowed to air dry. After, the mask should go back inside the plastic bag it came in. The filters should be allowed to dry. The pre-filters should be removed and shaken or blown out and replaced if they are heavily contaminated. Most filters also have a place on them to write in the service date. If in doubt, replace them. Filters are relatively inexpensive. I store the filters in a Ziploc bag along with the face piece. Some companies actually provide a nice fabric carrying pouch to put it all in but it is always advisable to put into the plastic bag, then into the storage pouch.