It is sometimes useful to know which Android apps you are spending the most time on. This type of tracking can help you see if you are wasting too much time on frivolous activities. Other than that it is also useful to find and remove apps which you do not really use.
One such app that can provide such information is App Usage Tracker. It starts tracking when you first open the app. It shows a list of apps sorted by the amount of time you have spent on them in a certain hour, day, week or month. Clicking on the app shows a graph with the usage time and duration. You can pick how accurate you want the tracking to be from 3 level: high, mid and low. Higher the accuracy, the more it will drain your battery.
Tried with: App Usage Tracker 4.3, Android 4.4.2 and Moto G