In about three hours, a qualified plumber should be able to remove your old water heater and install the new one. This time window assumes regular circumstances and no unanticipated complications. Start by removing any boxes or other objects from around the heater and the surrounding area so that the service professionals have unobstructed clearance to the heater.
If it's a gas heater, it'll probably take a little longer owing to necessary venting and safety procedures. The plumber will test and modify the settings when the installation has been completed and the tank is filled.