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.

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.