Kingsbridge Scarlet: trade 1 cloth for 3 VP
I cannot find an authoritative source, but according to my best reading of the rules and BGG's forums, you can make the trade once per turn, during the phase of your turn when you play your action card, as long as that event is in play.

My justification:


p. 7 of the rulebook indicates that "for long-term events, you may make the trade once per round." 
p. 3 indicates that blue event cards must be taken into account when playing your action card.


Together, I take these to mean that you can only make the trade once, but that you can make it every turn, and around the time of your turn when you are using your action card.

These threads on BGG suggest the same thing. One of the posters cites an official Mayfair answer, but it's not linked anywhere.