Resolves at market price 3 days after it last crossed the 50% line.  There is no time limit on this.

Some examples: if the price went to to 30% right after the open, and stayed there, then 3 days after the open it resolves at 30%.  But if after just 2 days it rose above 50%, the “timer” would restart when it crossed the 50% line.  It could later still resolve at whatever market price (but only if it didn’t fall below 50% for 3 days, or the timer would restart again).

Will Friday Jan 27th be “a sunny day” in London, England?