How many hours can you use a pressure washer before you need to change the oil?

I Have a 4 year old Troy-Bilt 4.5HP 2200 Psi Pressure Washer. ive recently bored out the Inflow/outflow of the pump increasing the water flow, put a high performance spark plug and put bigger jets on the carburetor. and i began using 93 octane Shell V-power Nitrogen enriched fuel. My pressure washer cleans better then my friends 2800 Psi washer. but my question is how many hours can i go until i need to change my oil, im curious because of my recent modifications im sure the motor is harder on oil.How many hours can you use a pressure washer before you need to change the oil?
Oil does not lose its properties, just gets dirty.That is main reason for filter on a car. Hope this helps.

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