Steve is pretty much correct. Sometimes the OEM specifies a different rpm.
For example, Briggs and Stratton generators specify no load at 3750. (So at full load it will be turning about 3600.) They call the speed loss 'governor droop'.
Some Briggs Vanguard engines on 'swamp boat' motors turn 4200. Pretty much that same engine in a lawn and garden application is rated at 3600.
On certain zero turn mowers that needed a little more 'oomph', the rep let us set fast idle to 3700.
A lot of push mower engines are set around 3200 to 3400 rpm. I think due to a limit in blade tip speed. This may be a government requirement, maybe. (as if a blade moving at 3200 rpm won't do as much damage as a blade at 3600 rpm.)
Some of the smaller 1 cylinder Honda engines get a lean surge above 3400 rpm. So I guess that is about their max where they are designed run well.