No no, do not wait for the light to go off. You will be wasting gas and your mpg
will suffer as the engine will be on all the time but your miles driven for that
period will be zero.

The blue light just says that the engine is warm (not cold) anymore. It has some
correlation with the catalytic convereter (CC) temperature needed to shift to EV mode
but it is not the same.

Usually in summer I get into EV mode after about 2 miles driving at around
30 mph. (4-5 mins). Now in fall as temperatures are lower I find it is
taking 1-2 mins longer. This is to be expected as the CC is warming up from
a lower temperature and needs some more time to get to the required temperature.