Replacement O2 Sensors for Frontiers

What kind of life expectancy are you guys getting on this dual & triple O2 Sensors? The wired Replacements are $120 each x three required through Nissan. At what mileage do I need to get concerned? My thinking is that gas prices being what they are, I may be able to improve my mileage somewhat. I just used some F.I. Cleaner and that worked some. The Flowmaster #942451 has added 20% to my gas mileage on my roundtrip daily commute. That pays for itself, a new stainless system bought and paid for on gasoline savings in 4 months. Thank you Flowmaster. Moderator can audit my gasoline receipts, please do

My Went out at 83,000 (about) Good Luck!!!