I can't answer with absolute certainty, but I wouldn't trust measurements of any free ROS after storage. There are plenty of available reactants in biologicals, so you're likely to see both a steady decrease in free peroxide and a rise in oxidation byproducts. Perhaps you should consider measuring the reaction byproducts in addition to the peroxide itself?

The freezer temperature (-20ºC) doesn't stop the chemical reactions where H2O2 are involved only slows them down. Biological samples for H2O2 quantification should be flash-frozen in liquid-N2 at the harvesting moment and kept in liquid-N2 until analysis or at least in a -80 ºC chamber, although in this last there could also be some oxidation of H2O2.

Thanks for your help!