For my need it in a hurry with max output gennies, I set rpm to between 60 to 63hz, volts to 240... These are the little portables that I lug round the plot to do the odd bit of electrical needed in a remote corner type work... they seldom run more than an hour or two at a time, but are run hard...
The bigger, less portable standby units, I normally clock to 51hz unloaded, 230V, and they run at a lower percentage of total output, but much, much longer periods....
Both ways they seem to last well... but to run hot and fast and heavy for long ain't a good idea in my book....
Cheers
Ed