On Stackexchange I found a great analysis of the Difficulty bomb that was adjusted in the Homestead release.
As you can see in the chart, even if people stop adding GPUs to the network today, it will make it very difficult to mine in about a year. Right now with 14 second blocktimes we make very little, imagine blocktimes of 5 minutes.
Also this is assuming static difficulty, most likely the difficulty will keep doubling every 2-3 months which will bring the blocktime to a few minutes sometime at the end of the year.
The author posted the python script, so if someone is good at programming they might be able to recalculate the result using todays difficulty of 42T and with the exponential difficulty increase with GPUs being added to the market.
Looks like the 2GB GPUs will be able to outlast this so there is basically no point in buying GPUs bigger than 2GB.
http://ethereum.stackexchange.com/questions/3779/when-will-the-difficulty-bomb-make-mining-impossible/3800#3800Difficulty on 2016-06-03 is 35.15 T and average blocktime is 14.99 seconds.
Difficulty on 2016-07-07 is 35.18 T and average blocktime is 15.00 seconds.
Difficulty on 2016-08-02 is 35.25 T and average blocktime is 15.03 seconds.
Difficulty on 2016-09-06 is 35.56 T and average blocktime is 15.16 seconds.
Difficulty on 2016-10-02 is 36.40 T and average blocktime is 15.52 seconds.
Difficulty on 2016-11-06 is 40.17 T and average blocktime is 17.13 seconds.
Difficulty on 2016-12-02 is 48.56 T and average blocktime is 20.71 seconds.
Difficulty on 2017-01-05 is 88.83 T and average blocktime is 37.88 seconds.
Blocktime over one minute in January 2017
Difficulty on 2017-02-09 is 249.8 T and average blocktime is 106.5 seconds.
Difficulty on 2017-03-07 is 679.3 T and average blocktime is 4.829 minutes.
Difficulty on 2017-04-02 is 1.753 P and average blocktime is 12.46 minutes.
Difficulty on 2017-05-07 is 6.907 P and average blocktime is 49.09 minutes.
Blocktime over one hour in May 2017
Difficulty on 2017-06-02 is 20.65 P and average blocktime is 146.8 minutes.
Difficulty on 2017-07-07 is 82.50 P and average blocktime is 9.773 hours.
Blocktime over one day in July 2017
Difficulty on 2017-08-02 is 219.9 P and average blocktime is 26.06 hours.
Difficulty on 2017-09-05 is 879.6 P and average blocktime is 4.342 days.
Blocktime over one week in September 2017
Difficulty on 2017-10-02 is 2.639 E and average blocktime is 13.02 days.
Blocktime over one month in October 2017
Difficulty on 2017-11-05 is 10.55 E and average blocktime is 7.443 weeks.
Difficulty on 2017-12-01 is 28.14 E and average blocktime is 4.962 month.
Difficulty on 2018-01-05 is 112.5 E and average blocktime is 19.84 month.