After Spa I think they should decide, then it should be more clearer.
You just have to look at both Red Bull drivers and then Alonso in the Ferrari to understand what needs to be done. Both Red Bull drivers are taking points off of each other (a bit like 2009), while Alonso has the teams full backing and is the clear number 1, so has the best circumstances to maxmise his points tally. If McLaren don't do this, then it's sayonara to the WDC for Lewis Hamilton.
But then they've been craving to get the Constructors title for so long now, if they implement this, then, their situation for the season can become like Ferrari's, with one driver scoring the majority of the points. They are only 53 points behind in the constructors, and not making a clear number 1 and letting them both fight for wins and podiums, then the motivation will be high for both drivers, and more points will come for the Constructors Championship.
So the best for McLaren would probably be to make a compromise between the two, don't leave it too late, but don't make the decision too early. If Button finishes the way he did in Hungary in comparison to Hamilton for the next two races. Then they should really throw all their weight behind Lewis Hamilton.