EKKO JG PERFECT AMOUNT OF DH STACKS

Hello guys, so i recently changed runes from ekko jg hail of blades to dark harvest and honestly i feel that its wayyy worseee than hail of blades, with hail of blades u just hit them, get that movement speed boost and run away till your E is ready again and BOOM dead, but it falls off late game and thats why i changed to DH, i was wondering, WHATS THE PERFECT AMOUNT OF DARK HARVEST STACKS BY TIME, for example at 20 min, i have like 11 dark harvest satcks, and whole game its like 1 stack/2 mins, is it bad? Should i just stick to hail of blades