Engine changes take anywhere from about twenty minutes up to around 6-8 weeks, or longer if they decide to change the wing(s) at the same time. This time can be reduced slightly if they take the plane out of service during the change. It also makes it easier for the mechanics.
Usually 5 or 6 longish test flights will suffice, but if they've had to de-coke the carburetors then of course a few more might be necessary.
Nice one. Made I laaarf!
