1TB is a phenomenal amount of transfer unless you're a CentOS mirror (or similar).
If you have a non-stop 10 mbps connection to your server, you cannot exceed 2.5 TB in a month.
If you have a site that's pushing, say, 10MB of content every load (maybe a big Flash game), that'll take maybe a 1-2 minutes to load on a user's machine.
The odds of you going over your bandwidth allotment for a given machine are small.
If you're really worried, setup some round-robin DNS and a couple droplets to host your site/app, so that you can distribute your load