Why is my iPhone 6 using up my usage time so rapidly?

Why is my iPhone 6 using up my usage time so rapidly? - 1

Your phone isn't. YOU are.

With no info about the apps you use and which ones are running in the background, that's all anyone can say.

Use some of these tips and that should help you

http://mashable.com/2014/10/01/how-to-save-battery-ios-8/#zEbewpFkH5qr

Assuming you've updated to iOS 9 then you may have Wi-Fi Assist enabled which will disable your Wi-Fi connection when the connection is weak, many people have had huge bills because of this feature.

You can go to Settings > Cellular scroll down and disable Wi-Fi Assist

It can also be the fact that you're not closing the apps fully so they're still running in the background using data, double tap the home button and slide the apps to fully close them. Alternatively you can go to Settings > General > Background App Refresh and turning it off.