With respect to others who fly complex aircraft and fast jets as my experience is limited to GA aircraft I do not think that having a mandatory 24 hour break or other set period is necessary. It all depends upon the complexity of the aircraft, the nature of the task that is about to be performed, the pre-flight planning that has been carried out by both the pilot and observer/engineer
There is a world of difference between a simple check flight in an aircraft that you have flown a few days previously to test one single item that may need rechecking at altitude on a CAVOK day and a complex 3 hour flight test to thoroughly evaluate the integration of a new EFIS, autopilot or FMS system in marginal and deteriorating weather.
Commercial implications can add to pressure that can lead to mistakes. If you have to deliver test results to EASA on test flight on the Monday before the guy goes off on the Friday for his 4 week summer break then this has to be taken into consideration. Then is not the time to rush from one aicraft to another.
Sometimes the pilot is fully up to speed with changing cockpits and from a GA point of view changing between SEP and MEP aircraft produced by Beech, Cessna and Piper let alone the various manufacturers from other countries is a matter of experience, pre-flight preparation and willingness to learn from others.
Flying a Diamond Twin with Diesel engines and Garmin 1000 or a Cirrus without a proper checkout by a pilot who is experienced and current on the type and a thorough knowledge of the systems is foolhardy.
As the number of older B/C/P complex aircraft is significantly reduced it is difficult for budding GA 'test pilots' to obtain experience in flying a genuinely wide variety of different aircraft but please do not over complicate the issue by setting unnecessary rules. The ASIs in most aircraft built in the last 50 years are colour coded and it doesn't really concern me if they are calibrated in mph, kt or kph but read the POH and make a note of the critical speeds and systems before you get airborne.