The 1000 GB is the maximum you can use if you left it pulling down 3Mbps all the time.
Or to put it another way, to use the 70GB you have been told you are, then if streaming video at 3Mbps (which is easily possible with youtube and the 720p/1080p options) then 53 hours of watching youtube content.
A film can be 1GB through to 8GB.
Each music track 5MB
So it can quickly add up.