They don't tell you, Francois, you have to keep track of what you use yourself I'm afraid. They have new plans though with shock free data, which means you use up all the data allotted to you and then it will stop automatically.
You can also check your phone to see if it has a built-in feature that will restrict your data when you reach a certain point. For example, my phone will warn me when I have reached 2.5 gb. When It reaches 3 GB, it will shut off my data. Something to look into.