In theory plastic will last forever, because the plastic isn't biodegradable. Aesthetics and green issues aside, the problem with uPVC is the components that make up the whole, they tend to be of poor quality and workmanship, hence even the leading manufacturers will only offer a 10-year guarantee, not a lot for a product that will 'last forever'. Got this from a Google search... http://www.guildford.gov.uk/article/870/Repair-and-reinstatement-of-windows