Any suggestions for a free app to help me focus on tasks?

Hi I was searching for a good app that helps me to track my progress on the maximum time I can stayed focus on a task. I think seeing this number would help me to motivate to try stay focused longer everyday until I can somehow train my brain to be more focused in general. I downloaded Focus keeper and be focus which are based on 25 mins interval and 5 minute break. Anyone has a better suggestion, a really useful mobile app they have used or is there a popular app you know?

Thanks :)