From Dark and Darker Wiki
Hp potions do heal less if they last less time the only exception to this is barbarian's potion chugger skill which is likely the cause of some confusion, barbarianspotion chugger skill doesn't reduce your buff duration but rather changes the duration of the potion through some other means which also doesn't effect the total health restored (plus adds 20% to the heal) rogues and no other class benefit from having negative buff duration at all