There are furnaces out there chugging away pushing 35-40 yrs. old and some that die after 15 yrs. or less. I would recommend though having it checked as far as heat exchanger integrity and operation of the safeties. As for what kind of service, burners pulled and cleaned, blower motor oiled (if oilable), blower wheel cleaned/checked for broken vanes and separation, vacuum it out, temperature rise checked. Most manufactures recommend yearly checks. As for when is it time to replace, well you could get all checked and cleaned up and it run great another 10 yrs. or you could pull the burners and discover they have rusted out on the burner faces not burning properly, or crack in the HE is found etc. etc. The best time to replace obviously is on your own terms when you want to. Just keep in mind in terms of efficiency, that sucker is a gas hog.