Some folks do consider O2 sensors to be maintenance items. Remember ShadeTree Mechanic on TNN? The guys that hosted that show now have Two Guys Garage on Speed Channel.
They suggested 40,000-70,000 miles for an O2 sensor. Earlier if fuel economy was going down.
It stands to reason as time goes on, the O2 sensor accumulates deposits. All motors use oil by their design, and oil deposits quickly foul an O2 sensor and cat.
When the O2 sensor accumulates deposits, it becomes sluggish. This can have a dramtic impact on the cat, which will get too lean or, worse, too rich a mixture for proper operation.
I used to have a 1990 Toyota 4Runner. In 1995 at 178,000km, I noticed a nasty smell from the exhaust while backing up to park at a mall. Also the fuel economy, never impressive to begin with, had started going downhill.
The dealer wanted an arm and two legs for a new O2 sensor, but I got a replacement at NAPA for 1/3 the cost. I noticed an immediate difference in fuel economy, and no more nasty smell from the exhaust.
I also had a 1992 Honda Prelude and although it seemed to be running fine, I also changed the O2 sensor in that the same time I did the 4Runner. I picked up 1 MPG, so I guess it wasn't too bad yet.
I would suggest if you're using 1 quart every 2,000 miles or more, you will experience short O2 and cat service life. It seems once the O2 acts up, everything else goes downhill quickly.
FWIW: I average 4,200 miles per quart on Mobil 1 0W-30, 6,800 miles per quart on Mobil 1 10W-30, in a 2000 GMC Sierra with Vortec 5.3. I plan on changing all O2 sensors in 2 years