Are you talking about free time pre- or post-quals? And do you include work that goes towards your thesis but that you "have" to do (e.g. for a conference or internal deadline) as free time or non-free time?

My experience (and I would guess many of my labmates, though I don't know for sure) is that quals are really easy to pass, you spend at most 2 weeks of your life studying for them, and otherwise you're just doing research plus a few classes. Stanford is an outlier in that it has particularly few class requirements compared to other top CS departments, but it seemed like MIT grad students also often started doing research fairly early on, from my perspective as an undergrad there.

Depending on your funding situation, your actual time spent doing research may be more or less beholden to what grants your advisor has to do work towards. I'm on a fellowship and so can do whatever I want, the only consequences being that if my research after 5 years is uninteresting then I'll have trouble getting academic jobs.