The general principle is to use a resettable counter. You maintain a timer that counts up or down at a fixed rate, this could be seconds, minutes or whatever suits your process. At each 'tick' of the timer you check to see if the PV voltage had changed by more than your defined tolerance, if it has, you load the counter with the starting value again. When the counter reaches terminal value, the period of stable PV voltage has finished.
Example in steps rather than real code because you didn't say what language you are using:
1. make a timer counting at 1 second intervals (use TMR1 and interrupts)
2. use an unsigned integer as the counter so it has a range up to 65535.
3. read and store the PV voltage.
4. load the counter with your 'auto-off' period in seconds. (600 for 10 minutes in seconds)
at each timer interval:
5. decrement the counter at each timer interval so it counts down each second.
6. read the PV again and compare with the previous reading
7. overwrite the stored value with the new one.
8. if the difference is more than allowed, go back to step 4.
If the counter reaches zero it means the 'auto-off' period has elapsed with no change in PV measurement. By changing the starting value in the counter you can make the period as small as 1 second or as long as 65536 seconds = 18 hours, 12 minutes, 16 seconds!
Brian.