How do pet skills work?

If I buy a pet skill in the cash shop, does my pet gain that skill permanently or will it wear off after 90 days?

Permanently. Think of the skill you buy in the cash shop as being a coupon. The coupon lasts 90 days, the skill is forever.

I now regret using my perm pet refund MP on Black Cubes. I assumed Nexon was being Nexon and thought the skills would wear off after 90 days.

Whoops.