Transition training in general aviation is necessary when you are going to fly with new or unfamiliar equipment or aircraft. It could be as simple as upgrading avionics or other equipment in an existing aircraft or as complicated as moving to a different make and model within the same category and class.
Due to the variety of options for transitions, there is no specific FAA syllabus for transition training so the details will need to be discussed and agreed upon with your flight instructor. For some transitions, however, your insurance company may require a minimum number of hours of training or even expect you to attend a formal training program with a company such as Flight Safety International.
There are no minimum number of hours for ground or flight training, and no tests, checkrides or endorsements required for transitions, except when the training is part of a formal training program.
For almost all transitions there will be some required reading in addition to flight training. Some equipment manuals can be very tough reading but sometimes you can find a great article or even a book that explains things better.
Transitioning to a new aircraft will cover the basic characteristics of the aircraft systems, normal, abnormal and emergency procedures, performance, and limitations, all while emphasizing how this aircraft differs from aircraft you have flown before.
The most complicated transition is from one aircraft make and model to another. For example, I got my complex rating in a Piper Arrow III and later purchased a Mooney M20J. Both aircraft were single-engined, low wing, with retractable gear but there were very few similarities beyond that! The Mooney had different avionics, a different autopilot, pull-type throttle, mixture and prop controls, the switches, gauges, gear handle and circuit breakers were all in different places. And the Mooney had speed brakes, which the Arrow did not, and this was a totally new system for me. In addition to differences in equipment, the performance, speeds, power settings and aircraft handling characteristics were not the same and of course, the operating procedures and limitations varied as well.
It took a significant amount of reading, training and practice to feel comfortable and competent flying the Mooney, perhaps more than I had anticipated. But it’s important to take all the time and training you need for your transition and not to set an expectation of a finite number of hours to get it done. When you discuss your unique training plan with your CFI its unlikely they will be able to give you a timeframe or a specific number of hours because a lot depends on your currency, experience, learning style and how much you study on your own.