If you are looking at ROS/RNS directly, you can't freeze tissues at all. In addition, if you are looking directly at mitochondrial function, no freezing allowed. If you are looking at things like lipid peroxidation or endogenous antioxidant activity, you can freeze but you should do so by snap freezing in liquid nitrogen and storing at -80. At -80, the samples will be stable pretty much indefinitely, but at -20, they are more likely to degrade. I wouldn't leave them at -20 for more than a month.

I agree with Rebecca. Snap freezing tissue with liquid nitrogen and then storing them at -80C is better. A standard freezer undergoes freeze thaw cycles, so the temperature is not consistent, unless it is stand-alone -20C freezer.

For oxidative stress markers analysis you can preserve rat tissues or  tissues homogenates at -80 C for not more than 30 days

For analysis of antioxidant enzymes the tissue can be preserved for three months but preventing repeated freezing and thawing(not more than twice)

Storage the tissue at -20 or -80 is dependent upon the experiment. For enzyme kinetics storage is not allowed. Storage is accepted for other experiments like oxidative parameters, protein measurement, RNA and DNA. But storage the tissue or Homogenate-add Protease inhibitor and freeze according to  Rebecca D. Powell suggestion.