Like once, in a sprint that took 5 weeks, the last week was scheduled for bug-fixes / quality assurance. I did nothing for 3 and a half weeks, finished my assignments in 2 days, then for a whole week I just fixed a couple superficial bugs here and there.
Reported time: ~ 170 hours.
EDIT: to expand on this point, the secret is in how you do the initial estimation. Hugely overestimate the small / easy tasks (explain with technical babble if needed) , then underestimate the difficult tasks. So not only will you secure lots of free time, but you'll also be that genius that finishes difficult tasks in 1 hour :-)
Heck, you might even be promoted to management; that's how all the cool kids in management are doing it :)
