What are the required hours for an MOVSM?

from AR 600-8-22 para 2-25
d. The overall level of volunteer participation and impact of an individual’s community service is key to determining
whether award of the MOVSM is justified. Although sustained community service is not defined by a specific time period
or number of volunteer hours (for example, 36 months encompassing 500 hours of direct volunteer service), approval
authorities will ensure the service to be honored merits the special recognition afforded by this medal.
e. The MOVSM recognizes service provided to a community over time; therefore, multiple awards of the MOVSM
during a single assignment or tour of duty are not authorized.

It's completely up to the approval authority. AR 600-8-22 specifically leaves the requirements vague and subjective so the approval authority can view the totality of the impact of the volunteer service to determine if it meets their standards. I've seen as low as 100 hours be approved, and I've seen 300 hours be rejected (told to resubmit when the volunteer hours reached 500). If your contribution has made a positive impact, write up the award and pass it up to your FLL. If they submit and it is approved, great! If it is denied, you should get some indication of when you can resubmit.

The regs requirements is very vague. For instance, I did over 600 hours to receive mine and I knew someone in a different command receive theirs for one specific event.