Depends how often you download movies and wither their in HD or SD.
An average SD movie takes 1.2Gb
on the other hand An average HD movie takes 1.7Gb.
So if you download a single movie saw every 3 days then:
At SD At HD
Gbs Left / GbS per movie 80/1.2 80/1.7
Days left 67 47
usage = once every 3 days
However as boosted stated you would want to keep some space for drives, system data files, updates etc.
So you have these options:
1) Clear your hard drive of watched downloaded movies and start fresh.
2) Download, watch; delete basis.
3) Use stream services.
4) Get a new hard drive (I'd suggest a 4TB Western Digital hard drive) either internally -...