Unfortunately I don't have enough reputation in this sub-stackexchange, so I will answer instead of commenting.
This options is not available in Android (up to this date), but there is an issue (which you can star*), which has unfortunately been marked obsolete with no reason whatsoever. Maybe it can be rediscovered if enough people star it.
P.s.: Here is a similar issue which has not been marked obsolete (yet).
Edit: Another trick (which does not really give you the desired effect) might be to use an app from your provider (these often show you how much data you have left).
*Makes it more likely that it might be implemented at some point.