Wiktionary
adv. 1 (context US informal English) For a large amount or distance; a lot. 2 (context US informal English) To a great degree; very much.
Usage examples of "for days".
By George, I went home and tore up a sermon I had been working on for days.
It was as though he had reached the crisis of a disease which had been for days gathering force.
Old artillery-men have said that, after being at work for days in the trenches, accustomed to the continued roar of the guns, a sudden pause in the firing will cause them intense pain.
Some times he wondered if she were not very lonely, not knowing that she, too, lived for days on that one hour.
They'll hang round a dead ceratopsian for weeks, gorging and then sleeping off their meals for days at a time.
Most of all he wanted to be healthy and fit so he could fuck her for days on end until she begged him to stop the delicious torture.
Well, if I pay attention to my body's signals, I can sometimes go for days without sticking my finger.