I prepped for the MCAT over the course of 2 months (mid June to mid August). On week nights probably just a couple hours a day. On the weekends, closer to 5 or 6 hours. We’re talking about 7 years ago at this point so the only thing I know for sure is the 2 months.
In terms of downside, technically you’re giving up a year of peak salary but of course then the question becomes if your peak salary would be different if you didn’t take the gap year.