If potion invention is slow, Harry must have gotten the light potion from a book, since I don't think there's enough time between battles to do serious potion research safely between classes and homework, even for Harry's 30 hours a day. If he can invent potions that fast, he potentially has a huge number of instant win conditions available (that's what I really meant, that rapid potion invention would be a huge pain in the ass to write around). I think at this point it's clear that Harry probably does know enough to invent potions, but not without probably months or years of experimentation per new recipe. If he didn't know enough to be dangerous he wouldn't have freaked out Flitwick.