And they're saying double that by next weekend if the invitations don't dry up again.
So assuming that the rate of growth stays the same at 10 million per week, it'll catch up to Facebook's userbase in a year and a half.
However, as the current rate of growth is limited by the fact that an invitation is required, let's say the current rate of growth doubles when the service opens to the public in, oh, let's say.... September to be conservative.
10 + (10 x 8) = 90
(750 - 90) / 20 = 33
33 + 8 = 41
To break down the math I'm using, here you go.
10 [million] + (10 [million] x 8 [weeks]) = 90 [million users by September]
(750 [million, FB userbase target] - 90 [million]) / 20 [million new users per week] = 33 [weeks to achieve Facebook's user base from September]
8 weeks from now to September at 10 million users weekly and 33 weeks from September onward at 20 million users weekly means that G+ will have grown to equal Facebook's current user total in 41 weeks, or less than a year, should the record of growth maintain itself through to September and (only) double when G+ is open to the public without invitation.