You can change it using the clock icon when you view the map prior to deployment for that battle.
15 is low but MBT tends to give turns in the 20 to low 30 range. Generally more & faster vehicles plus more lethal weapons makes for a faster game.
I have a WW2 going in the snow with lots of mud trying to take a row of hills thought 60 turns would do it, past halfway & made all of 500yds
