Thx guys.
@fjz , I am experimenting with whitening for around 4 years Look at other posts on my blog for different methods.
Correlation of time and longevity of "white" is wrong IMO. If you treat plastic with peroxide for a longer time , say 7 days , sun is not required, warmth is. However, molecules that are removed by a peroxide in the process leave plastic surface very porous and that is direct cause of "yellow" color coming back faster.
Only method is to fill porous surface with transparent paint for example. That will definitely slow down the process. However, you will not be able to whiten it again.