Depends. Where I am, the nearest FD that takes volunteer EMTs requires 36 hours of shifts per month, an 8 hour Saturday training every month, and attending a department meeting at least every other month.

My fire/emt up in the mountains near seattle requires 48 hours a month and around another 8 hours a month training (which can be concurrent with shifts usually). But that also includes sleep if you're not called. As others have said, it's dependent.

OP: my service requires 2 twelve-hour shifts a month + a monthly meeting. I had to complete a training packet before I could ride as a "full" EMT. Many services have stricter requirements; some require weekly shifts.

Maintaining the certificate also requires ~40hrs of continuing medical education every 2 yrs, which I accomplish via short 1hr courses every few weeks.