How long they usually take?

Ive NEVER bothered to do it before (tho HD is about 5years old) and it takes an age, like fookin days Prob been on for about 12hrs on and off (thankfully it pretty much carries on from where it left off last time) and its still not 50% thru

Is there any point to it anyhow? Does it free up any HD space? I could do with that...