I’ve not washed mine for about 2 weeks as I’ve been too busy (and where I park the washbay is full of crap from builders and subbies not reversing back far enough when backwashing).
Most of the the other drivers are washing theirs very regularly; I wonder if the new motors in our yard have anything to do with it 
My Dads fleet gets washed every weekend he says that he dont want them to leave the yard dirty 
and it is nice to keep the lorries clean
I don’t 
It’s done for me when I get back from a trip or if it’s a longer trip and it’s looking a bit grubby I just stick it through a truckwash
To be fair it’s a good quality paint job and never actually seems to look that dirty 
Still, I don’t like getting grimy when I’m clambering on the trailer strapping up 
Every day that the onsite wash is working I put whatever my day’s chariot happens to be through it. What amuses me is the drivers who can’t be bothered to wait for the wash but then park up and spend 15 minutes sitting stationary to satisfy their own misunderstanding of the company’s 15minutes for checks rule.