What type of lubricant should be used on PVC windows, and how often should it be applied?
Quality sliding PVC windows use rollers. Keep the tracks clean, using a slightly damp lint-free cloth. The rollers take care of themselves.

On cheap PVC sliders, I use paste wax after cleaning the slot with a slightly damp lint-free cloth.

You can also use spray silicone. Clean the track with a slightly damp lint-free cloth. Spray the silicone on another lint-free cloth and wipe the track down.

Don't use any petroleum lubricants on PVC if you want it to last.