Quote from SomeYoungGuy:
Back of the envelope calculations:
One tick level data packet can't be more than 50 bytes (10 bytes for unique instrument ID, 10 bytes for price, 10 for quantity, 10 for timestamp, 5 for venue, and 5 for checksum).
So if you were getting 10 updates per second on a very active insrument, every second for an hour, that's 50 x 10 x 3600 = 1,800,000 bytes or 1.8 MB / hr / instrument.
Let's say that you are a really hardcore trader, watching the 100 most heavily traded instruments wordwide, simultaneously, for 15 hours a day, 25 days a month. That's 1.8 x 100 x 15 x 25 = 67,500 MB. Or 67 GB per month.
It's streaming the HD pr0n movies that's going to bump you over 250GB / month.