Assuming a Month has 30 days & you download 24/7, then with 512 Kbps connection which means 64KBps download speed, here is the usage.
64 KBps means 3840 KB or 3.75 MB per minute download.
3.75 MB per minute means 225 MB per hour
225 MB per hour means 5400 MB or 5.2 GB in 24 hrs
5.2 GB in a day means 159 GB approx in a month.
Its bad that sucH policy is forced upon us, but even then, with 512 kbps Connection, you can not download more then 159 GB in a month anyway. With proper control, you can get all your work done in 100 GB limit on 512 kbps plan too. then again, what is it that You have to download? Even if you try Linux distributions etc, even then you won't be able to download more then 159 GB in a month.
At 512 kbps speed, 120 GB limit should be termed as fair. 250 GB for 1 mpbs connection.