if you keep moving and don't leave the pole lying about too long I can normally keep it going, first year I got caught out chatting to a customer and I left pole against the van, the brush was frozen solid against the van, I have seen me with brush in front of van with heaters going to get going again, exact temperatures I am not sure as I don't carry a thermometer. But at a guess -2 to -3
If you're using cold water then +1 degrees. It's pretty obvious that temps. below will freeze the water.
Last year I had the water flowing no problem but it actually froze on the windows. That was when I gave up!