The bolt on would kick in to cover whatever allowances you have exhausted. Ie if you've used all your data bundle then buying a data bolt on would then kick in and stay live until your anniversary date.
So if your close to your anniversary date then it wouldn't be worth buying a bolt on.
If your on contract then you won't be charged for going over data just throttled. However if you don't want throttled speeds then you can buy the bolt on, but any mb you've gone over will be deducted straight from the bolt on mb allowance.
