I recently did some experiments with Retrobrighting plastics, using Hydrogen Peroxide to bleach them back to their original colour. I wanted to see how temperature and concentration affected the process. Once I'd gathered the results I wanted to run another experiment to see the affect of time.
More Retrobright experiments
To perform the test I placed sample pieces of coloured plastic in jam jars with a 3% Hydrogen Peroxide solution in a water bath set to 40°C and then removed them one at a time after the alloted time had passed.
Similar to last time it was challenging to get a decent photo of the samples to reflect their true colour, but here they are. The "Top" Control piece shows the sun damaged darker colour, and the Bottom piece shows how it would have looked when it left the factory.
As you can see, the correct colour match is around 4 hours, or possibly a little sooner. It's likely to vary between colours and batches of plastics but it should give you an idea of how long the process will take. I'd recommend checking hourly to make sure you don't over do it.