I really don't think there's a one size fits all for this question.
It can really depend on the type and style of the main quest, the nature of the world you're exploring, the way side-quests are handled along with the way of exploring the world and the lore presented to you.
Too many different things can come together to make a good rpg feel drawn out or too short and missing depth.
Edit:
Also on consideration, one other aspect to complicate matters - A game can feel perfectly lengthy (or not) for your first playthrough, but then on later attempts at playing it, that perception can shift when you have to drag yourself through certain content to explore other areas.