When do I end a Major or Great Flare Watch?

Bill's rule of thumb: when you think it's time to end it on the basis of the usual set of criteria, just wait a couple more days.

N.B.: Here are some papers that show that it's not simply related to growth or decay of active region magnetic flux or helicity injection rate:

Smyrli, A.; Zuccarello, F.; Romano, P.; Zuccarello, F. P.; Guglielmino, S. L.; Spadaro, D.; Hood, A. W.; Mackay, D.
Trend of photospheric magnetic helicity flux in active regions generating halo coronal mass ejections
(Plots of AR flux vs time are in the top row of figures in the appendix. Vertical lines indicate flare/CME times.  )

Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana W.; Qiu, Jiong
Predictions of Energy and Helicity in Four Major Eruptive Solar Flares
(The four M and X flares studied in this work all occurred after 1-2 days of no significant flux emergence or cancellation)