Using Crighton's Law of Rootschat Extrapolation (1883)...
Since 1 August we have been recruiting at the rate of one member every 33 minutes.
If all carries on in a similar vein (and my guerilla tactics on another board this AM doesn't inflate the count!), then I reckon we would expect to welcome our 20,000th member at around 5pm on 6 August (BST).
cheers
Paul