Keep an eye on your API account usage by looking at the Statistics Graph in the Accounts System.
To access it, click the Check API Usage link on the homepage of the Accounts system.
You can also visit the URL: https://accounts.whatismybrowser.com/buyer/stats
API Access is metered differently for each end point; so make sure to choose the end point to monitor in the Blue Options/Select field.
You will see a graph similar to the following one, showing your usage for the time range you have selected.