Respirator LifeSpan..

they have a shelf life as well as a use life.
Certainly for organic vapours, change every year or sooner if you smell fumes. The outer partical filter should be changed as and when you can see it's getting blocked.

I use a respirator that's used by auto body painters. It comes in an airtight, resealable bag and has two replacable cartridges on either side of the mask that have a 10 hour life outside the bag.

thanks vapochilled. i'll keep that in mind

Changing your cartridges yearly (or monthly, or quarterly) is certainly OK to do, but may not be necessary. The general rule of thumb is once you begin to smell vapors inside the respirator (assuming you have a good fit), then it's time to change cartridges. This is driven, in part, by how much spraying you do, vapor concentrations in your work space, respiration rate, and where you store your respirator assembly when not in use. Keeping your respirator clean and stored with the cartridges in a sealed plastic bag can add months to the life of a cartridge; moisture will bring cartridge life to a quick end.
For particulate filters, once there is increased resistance to breathing (over and above the normal resistance that comes from wearing the respirator), then these filters should be replaced. The rationale for organic vapor cartridge life is essentially the same for particulate filters. From an occupational standpoint (just for comparison), I have seen particulate filters changed anywhere from 3-4 times for each replacement of an organic cartridge.

Gip Winecoff