well you need time so that gives you 2 choices :
Whichever takes more time .
To make it easier using it most times is also better , so using it asap to trigger the cooldown faster is also a good choice .
Well so : use it asap on first hard , or first couple of packs you meet .
I guess you could be using , or should be using it between bosses , like before 1 before 2 etc , to get most benefit , so it does recharge in boss , judging by gold timers its like 3 4 times per run amiright ? maybe 5 when its 20 minutes .
So you you have a precise time lapse when you MUST use it , you know which are best condition to use it . (also its nice to use it when mounting is possible)
So with all that staff shouldnt be very hard to choose the best moments . IMO