Hi Richard, I believe you will get lots of different advice on a topic like this. But here's my two cents. You mention above that you experience noise after a solid month of graveling and I believe this is quite normal.
In clean road conditions things will continue to work well for a long time, but if you ride off-road things deteriorate pretty quickly. Just for reference, I worked with a professional cyclocross team for several years and the bikes get a complete overhaul after every race. So that's about 45-60 minutes of riding, before the bikes are completely cleaned.
I clean and lube my drivetrain after every off-road ride and at a minimum I do a full cleaning of my bike once a month, usually more than that depending on the conditions.
If you ride a lot off-road you will have to do a lot of cleaning to keep things running smoothly. I would keep my main focus on the drivetrain, keep that clean and lubed all the time. The rest of the parts are much better shielded from dirt and grime.
Hope that helps.
Oh and don't use a pressure washer, you'll likely do more damage than good.
