1TB HDD: ~931,32GB or ~953671,68MB
Used Space on HDD: ~80GB or ~81920MB
Available Space on HDD: ~871751,68MB
85% of Available Space on HDD: ~740988,93MB
~500MB per Week
~1481 Weeks or ~28 years until 85% of the HDD fills up (with the previous data)
Of course this is theoretical. Real world is another thing

but that'll give you an idea.
This is only an approach, you need to find out how many GBs do you get left after formatting. then substract the used GBs to that number, then multiply the available space by 0.85 (which is the percentage you want to use), then divide by the number of MB per week you want and then you have the number of weeks, divide weeks by 52 to get how many years and that's it