Are the network transfer limits per-droplet? Or does DigitalOcean combine them all into a single bandwidth pool like Linode does?
Suppose I create two droplets at midnight on July 1. Each comes with 2 TB worth of monthly transfer. At the end of the month, one droplet has used 3 TB of outgoing transfer and the other one has used 1 TB. Did my usage remain within my monthly limit or did I exceed it by 1 TB?
As far as I know they don't combine them in single bandwidth pool. But you can verify with support team, or wait for MOD answer.
Also, DigitalOcean for now doesn't have utilities for measuring/monitoring bandwidth. You can't see how much bandwidth you used this month, or this week. Because of this, DigitalOcean is not billing for bandwidth overages.
How much long they will not charge is unknown, they will not charge until they implement monitoring tools. Anyways they will send out emails to all users when they do. That can be for month, year...
When they do that, they will charge $0.02 per GB over the bandwidth limit.