Could I use a pressure washer on a house with a well pump? Should I be concerned about the pump running for a long period of time i.e. 4 hours? half a day while we wash the outside of the house?
Most consumer-grade pressure washers require the input water to be at least in the low range of typical household pressure from a municipal water supply. It also needs to be continuous; if a well can't keep up with the demand, you'll destroy the pressure washer.
But, there's an option: operate from a tank of water with a pressure washer that can operate with a siphon feed.
Even small gas pressure washers typically use 2.5 - 3.5 gpm, so 4 hours of run time would require on the order of a 600 - 900 gallon or larger tank. But a gas pressure washer can clean a large area quickly, so you may not need 4 hours of run time. Electric washers use half of that.
You prefill the tank at a convenient time. You can also fill it with captured rain water, like from your downspouts, that you filter REALLY well.
While most consumer pressure washers can't operate from a siphon, there are a few that can. For example the high-end Karcher machines can, and they sell a siphon adapter kit for them.
If you can rely on your well for continuous volume at around 2 gpm, you could feed a siphon-capable electric washer without needing a tank or worrying about run time.
I do it. Your instinct is correct. I try not to 'exercise' the pump more than four hours a day. It runs a lot when Pressure Washing. Take some coffee breaks.
Experience showed that the well pump water pressure is set to match most "city" water's pressure which is between 50 and 60. Our pump was set to 50 and the pressure washer we purchased is 1750 ps1. We connected the pressure washer and it worked without issues.