Is there a minimum or maximum duration for the volunteer work?

The minimum period is 15 days (the trip from Quito to San Francisco takes an entire night and includes passage in a boat) and the maximum is determined by the duration of the visa which you are granted (3 months in the case of a tourist visa, but this can be extended). There is no time restriction on the part of FCSF if one is determined enough to complete the procedure of extending the visa.