At this point all that can be done is extrapolation and guessing for difficulty jumps. The probability of the network continues to grow at 20-40% every 10-14 days is very small. Right now it can and will happen due to the small / new nature of BitCoin, but that kind of growth simply can't be sustained for a long period of time . Eventually you'd hit the point where your rate of growth is faster than the speed at which new computing power is created on the assembly lines.
I think (and hope) that the network speed levels out to a gradual growth rate in a few more months (so probably around 500-600k difficulty), at which point the growth would likely be linear rather than exponential.
Fine!! Fine!!!! I said it was just an estimate. Here is the equation at 6.5 blocks per hour. shesh
210,000(when the change occurs) - 123,203(current block) = 86797(blocks left)
86797 / 6.5(average blocks per hour*) / 24(hours in a day) = 556(days till change)
556 / 30(days in a month) = 18.55(months left till change)
556 / 365(days in a year) = 1.524(years to change)
Here is a precise answer to your question assuming the average rate is seven blocks an hour from now(5-10-2011, Bedtime for a normal person in the U.S..A) till the change.
Hope this helped you
*6.5 blocks an hour is an estimate, if you find a more informed prediction just plug it into the equation above along with the current block number for a more precise answer.