I bought Pok?mon Moon on day 1, but I never got to finishing it. Not because it's hard, but because I don't have a lot of time. I do now, so should I finish my copy?
According to How Long to Beat, simply beating the game can take you anywhere from 21 to 46 hours depending on how you play. If you're onto completing the game 100%, then expect the game to consume up to 152 hours of your life!
For a RPG game, about 30 hours of gameplay just to finish the game seems to be pretty reasonable. It's just enough for the game to not overstay its welcome, nor does it feel too short.