Whichever retrac you chose, an hour of circuits would be plenty BUT to do the aeroplane justice and to safeguard yourself and pax it is essential to fully and properly understand how the gear retracts and extends and practice at least one emergency extension in the circuit.
I learnt this to my cost years ago - did my initial retrac training on a Mooney then did a proper full conversion to a Seneca. A hundred Seneca hours later I started to fly a Chieftain after a fairly comprehensive brief. A few months later whilst in the aeroplane on my own I had to pump the Chieftain gear down - no sweat, if I could not remember exactly what to do the instructions were under the emergency handle cover.
However, the 32 odd strokes that it required to get the gear down and locked proved somewhat problematic - as I pumped I couldn't see out the front, when I paused to look out the pump lost miniscule pressure so it became harder work! Had I done this 2 up as part of a trg programme I would have known what to expect and not started to worry circa the 20th pump that it was not working at all!
Stik