Our club checkout for different (simple) SEP types such as C152, PA28-160, C172 can range from 30 minutes to as much as is needed by the student to handle the transition.
One thing not yet mentioned is the insurance side of things. While both parties may be SEP rated, and therefore technically legal from a licensing standpoint, are they both insured on this aeroplane?
If the aircraft is a club one, they will probably insist on a club checkout, in which case flying as P1 without the checkout will probably invalidate the insurance and is therefore illegal from an operations perspective. If the aircraft is privately owned, most (not all) are run on the basis of named pilots. information on the number of hours on type is usually requested, and an appropriate checkout with an instructor is often advised.
For the sake of a relatively short amount of time, it's really not hard to get a check on a different type. Would the PA28 pilot know where to find the fuel pump on the C152? And would the Cessna pilot know what do do with the flaps in the event of loss of power on a PA28? And vice versa in each case? (Both trick questions, obviously, but if you don't know the answers the you can see how 30 minutes with an instructor is a good idea.)