This has happened to me for several play throughs, potentially since the latest update but I can't say for sure on that. Default end conditions are selected and normal speed is selected, which should have the game end when you hit 300 turns. But when you get to the last era it has an infinite symbol for number of turns left and we can play well past 300 turns. This has happened for three or four games so far. No mods applied, both myself and the other player are at the latest patch level on Steam. 

Here's a screenshot of what is seen for the number of turns left. The game ends when a different end condition is triggered, such as science or planetary. Any guidance on fixing this back up would be great. I have attached the last save from this game as well.

auto-save 5290.ctr