AMCAS asks start date (month/year) and end date (month/year) and average hours per week.
You could list start date & end date and leave hours per week blank and then detail in the description section:
Local marathon water station volunteer: description of duties, date, x hours.
Local soup kitchen Thanksgiving dinner: description of duties, date, x hours
Annual charity event, description of duties, date, x hours
Campus blood drive organizer, duties, date, hours