Shouldn't take more than an hour or two to get used to. Basically, your throttle controls the manifold pressure (MP), and you have an extra lever, the prop lever, to control the rpm. Instead of learning one number for each power setting (e.g. 1600rpm for base leg or whatever you use on your current type) you have two to learn (e.g. 16" MP, 2500rpm). You need to know what order to move the levers in (prop then throttle if increasing power, throttle then prop if decreasing it is a good starting point) but that just about covers it.
Of course a good instructor who knows the type will be able to give you specific pointers on the aircraft you are thinking of buying. There's a lot more to the theory than this, but you don't need to know the theory to be able to get started on the aircraft. But do read a good textbook for the theory, too, to get the most out of it.
Having said that, isn't the D20 a single-lever control??? In which case, there's only one lever to move, just like your C152 - as you move it forward, it increases both the throttle and the prop settings. If this is the case, the only real difference between this and a fixed-pitch prop is that the engine note doesn't change if you alter your airspeed.
Maybe someone who's flown a D20 will have something more specific for you.
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