You will be notified via email if you configured the notifications in Nelio A/B Testing settings page.
Your monthly quota is automatically reset every month. Meanwhile, the active tests will continue running but the tracking of visitors in your active sites will stop and will not count for your running tests.
However, you may increase the available quota when you need it through Nelio A/B Testing itself. Go to your Account Details page inside Nelio A/B Testing and there you’ll see options to Buy More Quota or even Upgrade Subscription.