Do You Know?
You can use your handset to track your data usage from month-to-month.
Navigate: Settings > Network and Internet > Data Usage.
In here it automatically gives you your billing period against your current used allowance. You can set limits to your data usage, to make sure you're on top of what you've used.
Navigate: Settings > Mobile Data > Current Period.
In here it logs total usage, you'll need to reset this on the 1st of each month to make sure you're getting an accurate reading. You can reset by scrolling to the bottom of the page and clicking 'Reset Statistics'. We'd suggest setting a reminder in your calendar so you don't forget.