This estimated, pooled bandwidth allowance is entirely based on existing Droplets as well as Droplets that were active at one point during the month. Each Droplet size has an allotted amount of bandwidth that contributes toward your accounts pooled allowance. You can see the Droplet sizes and amount of bandwidth associated with each from our pricing page here: https://www.digitalocean.com/pricing/
This allowance does take into consideration how long these Droplets have been or will be active. To explain this in a bit more detail; We currently bill for services at an hourly rate up to the monthly maximum cap of 672 hours (28 days). The pooled bandwidth allowance is also based on the same structure and will vary depending on how long a Droplet was active.
As an example, if you had a Droplet with a 2TB bandwidth limit active for 336 hours (14 days), your monthly pooled allowance for that month would include 1TB for that Droplet as it was only active for half of that period.
This allowance also assumes that any active Droplets will remain active throughout the rest of the month. This means that your estimated allowance can change if you are creating and destroying Droplets.
You can find more information in our Product Docs (https://www.digitalocean.com/docs/billing/bandwidth/) and you can view a summary of your estimated bandwidth usage and projected limit (https://cloud.digitalocean.com/account/billing).
Regarding the Droplet Bandwidth overage billing, refer here :https://www.digitalocean.com/docs/droplets/#bandwidth
Hope this helps!