It's best to use it when you kill the next dragon, because you get ability to go through zones in seconds without having enough damage (this ability has 2 h colddown). Then without having dmg or heroes you can boost your stats easily. If you can't kill dragon (100 zone boss) then it's a good idea to use concert when you get x3 bonus from concert comparing what you have in the moment.

Thanks for the tip, i barely get to zone 52, i usually focus on beastmaster builds or silver arrows elves

at every stage 70 rewards are dramatically increased.
70, 170, 270, 370.
Those are the stages to focus on doing things like that.
I should specify I mean after beating those stages.
471 grants the reward.

When my drum tech has everything set up right, about half a dozen beers in and im feeling ready.