It depends on the type of plastic. Materials such as ABS (which is what most plastic parts on a car are made of) are going to take a long time to break down in

It depends on the type of plastic. Materials such as ABS (which is what most plastic parts on a car are made of) are going to take a long time to break down in the sun. After 15 years in the sun it may look a bit faded, but structurally will be fine. (I'm talking about unpainted plastic of course)