It's usually not a "percentage" thing. If you're working with a company, it typically means they own the rights to any work you do for them, including the design.
This isn't a bad thing, but you DO need to make sure you get paid properly for it and have a contract set up that spells out of the terms of the agreement. They're the ones who should be providing you with one.
Commercial rates (where you don't retain ownership of the work) are much higher because it's assumed they're going to be making money off your designs without compensating you later. Depending on the size of the company, I'll charge 2-4x what I usually would.
Even in cases where you retain some ownership, it's still not a percentage--it's usually "what you can do with this design". Like they may enable you to post pictures of the character on social media, but not print the design on merchandise to sell. That sort of thing.
