Short answer: With normal daily use, a good vinyl skin lasts for months without lifting or fading. Most people change it because they want a new design, not because it wore out. It peels off cleanly whenever you want to switch.
What wears a skin out
Edges and corners take the most rubbing, from pockets, bags and hands. That is where a worn skin starts to lift first.
A skin applied to a dusty or oily back never bonds fully, so it lifts sooner. Most early lifting comes from the install, not the vinyl.
Constant friction from a tight case pulled on and off every day also wears the edges faster.
How to make it last
Clean the back with an alcohol wipe and let it dry before you apply the skin.
Press the edges and corners down firmly after applying, then leave the phone for about 24 hours before using a tight case, so the adhesive settles.
Wipe it with a soft dry cloth. Avoid strong cleaners and solvents on the printed side.
When to change it
Change it when an edge keeps lifting after you press it down, or when you simply want a new look. Peel slowly from one corner and the old skin comes off without residue.
Quick questions
Does a skin fade in sunlight?
With normal daily use a skin holds up for months without fading. Leaving the phone in strong sun for long periods, such as on a car dashboard, will shorten that.
Can I reuse a skin after removing it?
No. Once peeled off, the adhesive and the cut shape won't sit perfectly again. Each new design is a new skin.
Will a skin last longer under a case?
Yes. A thin case covers the edges, which is where skins wear first. Wait about 24 hours after applying the skin before putting a tight case on.