I recently replaced a broken small window with Lexan polycarbonate. Lexan is reported to yellow after 10 years. How bad is this likely to become, how quickly -- will I have to change the window again in 10 years to be able to see clearly through it?
About 10 years in Florida, if given an anti-UV treatment. Here is an old GE technical report which gives details http://www.bristolite.com/interfaces/media/Tech%20Report%20GE%20Lexan%20Polycarbonate%2010%20Year%20Aging.pdf
A photo of the situation would help. But if you are "repairing" the meter itself then the better advice would be that you get the meter replaced instead. There could be some concerns raised with tampering with the meter if its seals are broken.
On the other hand if this "window" is a separate thing from the meter then why mess with Lexan if there is a clouding concern. Just replace it with some good quality thick glass will stay clear for a long time.
It depends on climate and whether the Lexan was treated or coated for UV exposure. There are greenhouses that go far more than 10 years and they aren't yellow. For untreated Lexan the yellow is on the surface and can be polished off. My generic Lexan window on my chicken coop is about 5 years old and not yellowed that I can see. My climate is very sunny for 4 months of the summer and mostly cloudy the rest of the year, 48 degrees latitude.