I've had IBTs last as long as an 30 minutes.
On my older, Win98 500 MHz 256 RAM (iirc) box, I had long IBTs. Part of it was self inflicted. I had railed my empire and set most, if not all, of my workers on Automated improvements. I had a fair number of workers. Somehow I had a stack of workers and I watched as the computer took each worker, one at a time, and tried to determine what improvements could to be done with that worker. If no tiles could be improved the worker was moved to the nearest city and did nothing. Seemed to take several seconds for each one, at least 5 seconds and maybe as long as 15. After that I fortified my workers outside of cities and the IBT went much, much faster.
More recently, and on a better PC, while playing CCM on a CCM-Huge map, I had the IBT take close to an hour when it was late Industrial Age and the AIs were building a lot of units and fighting their own wars. There were still around 20 to 25 (out of 31) AIs in the game, too, because I had a rather slow start. But CCM is a mod and that is not what you are playing.
So, to second what vmxa said, just wait it out. It will come back.
Though, I would save the game at the end of your turn, in case something else causes issues with your computer (BSOD, power cut-off, soda spill, two year olds pushing buttons, etc).
