1995 Nissan Altima

I have 282,000 miles (yup!) On my Altima, and was wondering why my Check Engine light has remained on for the last 100,000 miles? I have had it checked out and have spent over $200.00 at the dealership changing out O2 sensor's (that's what is says is bad) but each time the light comes back on with a different O2 sensor reading bad. I have changed out 4 O2 sensors all while the car is running great. The dealership said it could be a thousand different sensors (that I would have to replace one at a time). No Thanks. What's the deal? It all started at the 190,000 mile mark. Could there be an indicator telling engine to change out all sensors at 190,000 miles? When should I have my timing belt changed? I have a manual.

thanks,

Dan