Well, you have about 100 hours a week to allocate, assuming you spend 9 hours a day on some mix of sleeping, eating, hygiene, etc.
I was in a very difficult major, spending about 60-80 hours a week just on class, problem sets, studying, etc. (sometimes more!) With the remaining 20-40 hours I committed (on average) about 4 a week to volunteering, 1-2 to shadowing, 2-4 to teaching/tutoring, 4 to research/other jobs and the remainder to leisure and miscellaneous EC activities (a couple of clubs, a sport, etc.) I took prereqs each summer that wouldn't fit into my major schedule, did MCAT over sophomore summer, and also ramped up the other activities during that time.
If you start these things early in freshman year, that'll get you to pretty respectable numbers of hours when it comes time to apply: 500+ volunteering, 100+ shadowing (IMO too much, do something else), 500+ teaching/research/etc. The key is to commit to something early, carve out enough time to do something meaningful, but not too much that you compromise your grades. Back in my day, the common wisdom was also that med schools like to see continued involvement (as long as it's productive), which is also why getting started early helps.