As said allready, it's up to the instructor to how long it will take. I just finished (today actually) my convex onto a baron 55 with no previous retractible gear or twin on my license at it took 4 hours. I had a VP endorsement though.
Most of the engine management stuff is in the POH for the aircraft for all the numbers, 55% power at such and such MAP and RPM, 60% and 75%. Most guys I know take a piece of paper with these numbers on them and stick it on the dash somewhere for easy reference. The rest is just good common sense like not to decent at 1500 fpm with less than 20" power but rather at 500 fpm at 20" and so. It's better for the engine and more comfy for the self loading freight
These things now fly a little faster and need a little more planning ahead that a C172. If you do that there should be no problems.