No way to know as Apple tends to kill a product before it's totally dead. But if I was to guess... 2 years, 3 on the outside. Once the volume of the retina is high enough and the costs are lower then you'll see it. Most likely it will happen when the 13" becomes retina only. The 15" would probably stay customizable and that would be the final year.
I finally decided to order the 15" hi res, non retina so I could swap in my 480GB SSD and pop in 16GB memory. I would love to have retina but I'd rather have retina on a high res or 17" MBP instead of the resolution they chose.