If you're talking about a Holding Deposit, there is none. The guest pays in full when they book.
If you're talking about a Security Deposit, you can set an amount for a security deposit (to protect your interests). Do this through Host > Manage Settings > Calendar and More > Pricing Settings. Airbnb will put a hold on the guest's funds for the amount of the security deposit. After the guest stay is complete, if you make no claims before the next guest checks in (or within 14 days whichever is earlier), the hold will be released. Actual funds do not change hands unless a claim is processed.