Net, net bottom line:
Longer in the summer than in the winter
Longer in a well-insulated cooker than a non-insulated one
Longer if your cooker has a PID controller than if it doesn't
Longer when you're smoking than when you're grilling
Longer if the wood in them is of a higher heat-producing "breed"
Longer if you cook out less often
Don't agonize or get OC over pellet usage. Pellets (whether purchased in a group buy for a few cents a pound or shipped out full retail from an online source) are a reasonably-priced heat source. Less price volatile in some respects to the vagaries of wood treatment supplies and petroleum.
Hub