I'm working on a 2D platformer and I'm trying to decide how many levels to have. It's an Android/iOS app. Each level is a little bit less than a minute long but you have to play through them at least three times to find all the unlockables. How many levels make it worth the $0.99 to the players? I want to have lots of levels, but I don't want to have any "filler" content.
It really depends on your pipeline, content, and time of investment. if making a level is difficult(for example, having to write a level all in notepad), then your going to need to invest more time, and if you get to your deadline, then that's how many levels you have. where as if you have a very easy to create level pipeline, and plenty of content to make it interesting. than you can fill your game with tons of levels, while still meeting your deadline.
at the end of the day it's about how much time you want to invest, if you have to go over that time to ensure you have enough playability to justify your asking price, then you should re-think how you are building your levels.
In my opinion, your best bet is to distribute the game to some friends/family and get their opinions, do they feel the game is too short, not enough variety, or potentially some other problems, it's always a good idea to get feedback before you push it to the market, just be sure it's honest/real feedback.