I guess you mean the "flow temperature".
Modern condensing boilers are actually slightly more efficient at a reduced flow temp.
Rather than have me rattle on, have a read here...........
https://www.theheatinghub.co.uk/articles/turn-down-the-boiler-flow-temperature
It surprised me to learn that max. efficiency can be around only 45 degrees.
As for the length of time the boiler is on... really a matter of trial and error. It will vary immensely for different properties.
As I see it, if it's on for longer but with a lower flow temp, then you are surely going to be toasty for longer, but with no extra cost. (Theoretically.)
Following the principle of "energy in = heat out", I think we're just fiddling around the edges here.
In older, less insulated houses, a lower flow temp may not even be enough to heat sufficiently.