Only really dumb RC would close the finishline after 20 minutes in a handicapped fleet with rating ranging from say 100 ro 129.
Ditto. A 20 min time limit would make sense for a one design race, and not a very long one, depending on the wind.
A very common time limit is the sunset or sunset minus time to shore, when the finish line is far from shore.
When many races are scheduled for the same day, we use both a fixed time limit (t minutes after the first boat) or the sunset, whatever happens first. Safety and common sense dictate that all boats return to shore before nightfall.
The number of minutes "t" is a function of the course length and the slowest boat speed people are willing to sail at. The RC should double checks t during the skippers meeting and adjust it if necessary.
