There are to many variables to give you an accurate answer really..outside temp, starting water temp, how much hose out, what flow you are using etc. I would advise you to ring them with a list of questions you have and on how the best way to run it.
Personally i have the heater on full whack and turn it on about 10 mins before i start cleaning with it re circulating back to the tank between jobs and a flow rate of about 45-50. Ideally you dont want to keep turning it on and off during the day as it takes a lot of battery power to start it up so best to just leave it running through the working day and switch off when the day is finished. If your coolant reaches a certain temp I think its about 80c or thereabouts then it will do an emergency shutdown to protect the heater, this wont happen if your circulating back to the tank, you dont really want to be stopping the water flow for to long.
Don't expect high temps this time of year as there is a lot of heat loss through the hoses , make sure your batteries are kept in top condition ideally they will need charging every night to keep them at their best performance.
You should of got a manual with it there are a few pages in there on examples of temps achieved (if you didn't i can e-mail you one). its a bit of trial and error to begin with but you'll get the hang of it pretty quickly.