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You can get an approximation on your phone. You can also go to your account and choose e.Care > Billing > view data usage > and choose a period to view. This should give you all the detail you wish.
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All the aforementioned options are great in fact. Obviously, there is no such option to know your data usage instantly. If you are using any network adapter, you can login to your account and see the network usage alters in every instance.
Usually, you might get sms describing your usage and the balance when you stop the data usage on your phone. Again, there is no option for this purpose.