Any double Glazers on Here?

Guarantee wise:

The plastic itself: There's no much to really go wrong apart from a bit of discolouration if your really unlucky.

The locks/hinges on the window: If they are kept clean and lubricated they will last a long time.

The locks/hinges on the doors: Like above, if kept clean and lubricated they will last. As the seasons change you may find in time to come the door catches the locks on the frame. Depending on what hinges they use will depend on how easy it is to adjust the door.

Glass: This is a big hit or miss area, most of the time the glass will last and last. But on the odd occasion you might find a glass unit has broken down. It's sods law that if you don't have any guarantee then you'll get a broken down unit or two.

[tigersaw]

from experience:

A bad fitter can ruin a good window. Check all windows actually open, that the night latches work, the locks work. Check draining channels arnt blocked by bad fitting. Make sure the window is actually held in by frame bolts, not just mastic and foam.

White upvc can yellow after prolongued UV exposure. Leaded inserts can lift inside the sealed unit after prolongued heat/sunlight.

Check glass for roller damage, lines or kinks that are in the glass, not something that will polish out.