You can run it non-stop over 11 years (100k Hours) if you spend less than ~2.6 years (23k hours) of that at OPP5. Only OPP5 would limit power on hours to ~5.7 years (50k hours) and any OPP variation ~5.0 years (44k hours). And these are the TI warrantied values at both nominal voltage and clock for that OPP, so I dont know how much overclocking at OPP5-voltage affects these numbers (speed has less effect than voltage, but maybe still some).
These numbers are from page 123 of the OMAP3530 datasheet.
EDIT: And the datasheet only says about OPP5 vs others, so you should be OK running it 100k hours at OPP4...
(And this information is all for the 600Mhz units.)