How long do I need to charge my device?

Download a battery app from the Google Play store. I recommend "Battery HD" by smallte.ch. This app will tell you when the battery is done charging and then you should unplug it to avoid overcharging.

There's also "Battery Doctor" by Cheetah Mobile, but that app asks for and has access to too much personal information and they advertise a lot of their other apps through it that you don't need. I'm not sure I trust Cheetah Mobile. A battery app doesn't need to view your photos/files or info about your calls.

The Google play store is not regulated as tightly like Apple App store, so some apps may be malicious or privacy violating without you really knowing. It's good to download an anti-virus app as well, like AVG or Malwarebytes.