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There are two phases for the limitation, first is a warning that will tell you about getting close to the limit, then there is the actual mobile data disabling phase,which you can set just below 300 MB.
You can set google play app to auto update only when you are connected to a wifi network. You can also restrict background data for apps that use a lot.
The limit can be changed by dragging the horizontal bars. One for warning, other for limit.