Yeah, HowLongToBeat really helps to give approximate idea about the length of the game - I tend to check (once you go to said games' page) Completionist/Average and Main+Extra/Leisure combination to figure out what it will take me to finish it (I'm not complenitionist, but I do tend to do most of stuff ingame, and at very leisurely pace). Depending on the popularity of the game, data is pretty solid (i.e. Witcher 3 data is based on 22K+ entries).