@Pam332 there is no 'normal' charge as there are a myriad of services provided by management companies.
For just management 20% is a number I have seen bandied around. So Airbnb gets its 20+%, the management company gets their 20%, then the cleaners and repair guys get their slice and there's not much left for the owner!
It really depends on what services you are offering and what others charge in your location. You need to do your market research for your co-hosting business to understand what charges you should offer @Pam332