Hey Mark
This pressure washer has been designed for light commercial use, so i would say 4 hours continues use if i were to take a guess!
My demo machine has been fine, as long as you look after the machine in terms of servicing... make sure the water strainer is clear, fuel filter replaced once a year, pump oil topped up and of course de scaling the machine on a regular basis.
When you add up the cost of an inline heat exchanger... average price £450 +VAT, then a 400psi pump and fitting...
£500.00 that's a grand... and even then the best inline heaters are never going to provide you with continues hot water as they are constantly having to heat the flowing water.
This method is the most efficient, cost effective way of adding high pressure and hot water to your portable extraction setup.