You'd have to edit the scenario. It's located in your civ install
"Sid Meier's Civilization V\Assets\DLC\DLC_01\Scenarios\Mongol Scenario\". Edit it with the scenario editor as part of the SDK. There's a textbox for "Max Turns" where it's set to 100. Just change it to whatever you want.
You shouldn't need to do it though. Additional turns are only going to allow the civs to build more troops, which is just going to slow it down more, and make it take longer. That scenario is about momentum and doing more with less.
Edit: If you need additional help, here's something I quick wrote up about it in the scenario thread:
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.