I think you mean the autoturn thing?   go to options and turn it off...

If you mean the turn limit, that is moddable.  You should be able to change it in Elemental Defs.  

Look for this line (line 314) in the file C:\Program Files (x86)\Steam\steamapps\common\FE Legendary Heroes\data\English\ElementalDefs.xml   

<!-- ** turn limit, and game pacing effect on it ** -->

<TurnLimit>800</TurnLimit>

Change the 800 turn limit to whatever you want.

If you mouse over the current season (or in the Immersion Mod, Month/Week) it will show you what turn it is.

Everything that has a beginning has an end

-The Oracle

And things that don't have a beginning don't have an end.

Having the highest score on the last turn is one way to change the game.  There is a bar on the victory screen that shows you how many turns you have left.  

Thanks StevenAus.

Next game will have that turned off in options as Smeagolheart writes. I don't like to feel stressed by time-limits in games, that's why I love turn-based.

I didn't realize there was a turn limit. Is the only way to change it by modding, or is there an option somewhere in-game?

I tried setting the turn limit in the xml to 9999 but it didn't change when I tested it in game, anyone know why?

If you were overwriting elementaldefs.xml, it doesn't always mod as easily as other files. Heavenfall has a post on this somewhere. I have tried to change elemental defs and they don't take (I tried forcing quest spawn).

And well just who cares about that score anyways. It is not even recorded by anything...