during a normal run in DC, assuming you don't get killed in-between, it only takes about 5-15 minutes to complete each stage. You'll eventually sort of get an feeling for how the stages are constructed and after you get some upgrades you are encouraged to avoid enemies and rush through the stages as fast as you can. Eventually, should you beat the final boss, you unlock higher difficulty levels and repeat the process a number of times (but keep all your unlocked abilities)
HK takes an easy 20-30 hours at least, quite a bit longer if you go looking for all the collectibles. Also, DC isn't really a Metroidvania. Think more Castlevania 3 but the branching paths are not accessible until you beat some bosses