It can be as little as an hour (some will claim to have done it in minutes). It helps if you can familiarise yourself with the panel layout and any variations in equipment, specifically radios, transponder, trim controls etc before you fly the aircraft. Note additional equiment such as autopilot, panel GPS etc that you may not be familiar with from the 152.
The rest of the process is handling differences and getting used to an increase in speed. A diiference of 20 knots can increase your workload if you are not prepared for it. If you opt for the PA 28 pay special attention to the fuel selector location and management of same in flight.
If carrying pax in the rear take a flight at max weight with an instructor to experience the difference and remember that some fields you got into and out of with the 152 will be off limits for a heavily laden PA 28 on a hot day.
Otherwise enjoy and best of luck