They scale with the commanded (S) speed..
So if you set it to 30 seconds for 0 to 20000 RPM, then if you do S2000 M3 it will wait 3 seconds (1/10th of 30 seconds because its 1/10th of 20000 RPM requested)..
I trust I can come up to speed inside 15 seconds regardless what speed I ask for.. I can't guarantee that that my acceleration is perfectly linear, in fact I know its not..