It is all a function of load. Load produces heat. And heat is what drives the turbo. C Lectric pretty much nailed it.
BUT....to answer your question....as it relates to MY boat. It happens about 1500RPM. You hear them starting to "sing". And then the props and engines start to dig in. It is a wonderful noise. I honestly believe that in most of these applications, the RPM will be similar.
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B