At 1500 RPM, the generator output is around 4-5 kW, depending somewhat on the engine size.
So, at 1500 RPM with 5kW output, it will take just over 1 hour to collect 6.5kWh (6.5/5 hours).
For reference, at 1700 RPM and cool temperatures, the output is around 7.5kW.
So, it will take less than 1 hour at 1700 RPM.