Great game imo, enough so for my first comment !
First of all, the gap in times surprised me at first, but let's face it : when you have 15 or 20 heroes, it is physically impossible to keep them busy with 20seconds quests. So having the 20minutes ones behind (10minutes with buff), and the 8hrs ones for when you need a break, is actually a really, really smart way of splitting the player's options. This is for the gaps. Short : if the gaps in quest durations surprise you, keep playing for a bit.
Also, I love how you take a lvl1 hero, tell him "You'll be a man, my son", and stuff him with monster gear (Power stat is the key to self-sufficiency for your twinkies), in order to take him to 20sec quests again and again ; so that when the upper levels come back from their long quest, the newbie is actually the new monster.
And last hint (really a beginner tip) : the max quest level is [Max hero available] + 2 if there is any available hero... or [Max hero level] + 2 if none.
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