I've only used the third reef two or three times but have been very glad to have it when I did. First reef goes in around 18 knots. Second and third reef go in when the helm gets over 10 degrees and stays there. That is usually around 30 knots for the second reef and a bit over 40 for the third but depends on point of sail.

I chose a third reef instead of a trysail knowing that I would be risking the integrity of the main in high winds but leaving room in my sail locker for a second headsail (I have a 100 and a light 135 in addition to a heavy staysail) and a second spinnaker.

Choices.