when i worked in the woods, i used about a a chain every 4 weeks, and a bar every 3 chains... mind you i didn't dull a saw very often, and they always got filed every day at noon and after work. btw, i'd burn atleast 5 gallons of fuel a week, and put up 30 cords of 8' stud wood and 2500-5000 boardfeet of logs.
after using a 18" bar, i cannot use a shorter bar in the brush. cutting red spruce, you need the reach to limb the tree when your waist deep in snow and brush. when i cut firewood with my father, when i'd run out of gas he'd pass me his saw off the skidder ( partner 5000 with 15 1/2" bar) and i could not use it in the brush, if your used to a long bar, the shorter ones are too easy to kick back.