I don’t think there are any rules, or a right or wrong answers…
I can turn mine on and in 5 to 10 min max it is hot enough for a nice shower… the water continues to heat even while showering and by the time I am done with the shower it is very hot and I have to adjust the temp with cold for rinsing, and the water is hot enough to last most of the day with light use… that serves me well and has for many years now…
I doubt mine ever gets to its full turn off temp which is scalding hot (dangerously so for children) in these little and inefficient heaters…
I don’t do it this way to save cost on electric or propane although IMHO it does, I don’t like to waste energy needlessly be it my propane or the CG electric… the advantage of DSI…