I've found that with my 560, temperatures is a huge concern for making a overclock 'stable'
I used to never be able to get anything stable in OCCT over 910MHz, now that I modded my chip and heat spreader, so that the max temp in OCCT is 64C, I can jack the MHz to 970MHz @ 1.087V with no errors on a 15minute OCCT test... Currently I'm running 1.050V @ 950MHz fully stable, max temp 62C in OCCT 20minute test (15minutes of actual testing, 5minutes of idle = 20minute test).
Honestly, run OCCT with error check, I like it because it'll tell me if I have errors. I'd say you only need to run it 20minutes total (15 of testing) to see if your clock is stable. Run heaven 2.5 benchmark too. OCCT will give you a good high number of the max temp your card will ever reach too...