RFC 1912 define a valueset between 20 minutes and 12h,
This is wrong anyway. RFC1912 recommends this range for the refresh interval of secondary nameservers (for people still using this ancient concept).
Regarding the TTL for entries, RFC1035 recommends 1 day and down to 1 hour when needed. There is however no rule which forbids values way lower. Lower values only increase load on the nameserver responsible for the zone, so only one self is harmed by choosing a very low value.
Regards Michael