That's up to you and your co-host to decide, and depends upon how much work they are doing for you. Rates seem to vary from 7% - 50%.
Questions to discuss:
Are they handling all the bookings, replies, emails, etc for you?
Are they handling the cleaning? What about simple maintenance?
Are they on-call for the guests?
You can pay your co-host a straight percentage and keep the cleaning fee (if you charge one), or give them the clening fee and a (usually lower) percentage of the booking fee.
We pay our co-host a straight 40% (his preference) and we keep the cleaning fee to offset that cost. He handles all the cleaning, bookings, emails, phone calls, everything except paying the taxes, etc., which we obviously have to take care of.
Hope that helps.