Android: Frequency tracks the amount of time you spend in each app on your phone. You can see where your time is going if you need to track time-for certain applications so you can bill clients, or even if you're just curious.
Frequency lets you see what apps you use, when you use them and how much time you spend in each one. You can set the time frame for today, the past week, the past month, the past three months or all time. You can also view all apps or only those that you have used within the time frame you set. The basic version is free. A Pro version adds the ability for Frequency to start when your device starts rather than you needing to start the app manually. It also allows you to change the frequency at which apps usage is checked from the default of 30 seconds.
Frequency: App Usage Tracking (free) [Google Play Store]