When signing a car rental agreement, most rental companies will request a security deposit, sometimes referred to as a credit hold, which will serve as a warranty for the car rental company. The requested deposit amount will vary depending on the vehicle´s category and rental period as well as on the rental company itself.
When using a credit card, the amount will not be charged directly, only put on hold. Once you return the vehicle, the rental company will request the hold to be removed. The wait-time for the hold removal to take effect, however, will depend solely on your card´s network.
As for cash and debit security deposits, the amount will in fact be charged by the rental company but returned after the vehicle itself is returned.