At what point is the app most useful, i.e when does it become meaningful for the user? It doesn't HAVE to be in days does it? Look at Dropbox, their free tier

At what point is the app most useful, i.e when does it become meaningful for the user? It doesn't HAVE to be in days does it? Look at Dropbox, their free tier lasts for however long it takes you to fill up 2.2Gb, 'cause once you get to that level a) you know it works well b) you're using it regularly so upgrading is barely an issue because you've already seen the worth. If it's a photo app you could say 50 images, they have a certain level of investment into the app so it becomes useful. If it's a game, when does it become addictive? level 3? 50,000 points? 40 in-game hours? I think users find 'use' in apps at different rates. By cutting them off after x days - you essentially lose them. Let them carry on under a certain threshold until it becomes meaningful to them, they'll upgrade to a paid plan. Let them mature to the point where they are most likely to upgrade.