Most of the time I use an app whenever it gives me any kind of value back. Value may be in form of enlightenment or joy, e.g. when it entertains me or is somehow funny. Usually I stick only couple of days to these kind of apps.
Even more important to me is that the app can provide value over and over again; for example by supporting my daily tasks(Todo list), personal development(habits or meditation), organization (calendar), communication (WhatsApp) and so on.
@aleksandr_lanin Headspace and Streaks, but I used Streaks only for couple of weeks and fell back to my old school pen and paper :D Headspace is awesome though, they also have this "streak" component to build up a habit for meditation. Anyway I think building up a habit through an app is a powerful way of maintaining users (see "The Habit Loop")