is there a setting for how long troops live after going into map room?
I'm not entirely sure what you mean by how long they live.
I think you mean that if you make them live longer, or shorter life spans it controls how many troops need to get to the map room to take it.
If that is the case, they don't "live" and accumulate inside the map room, essentially "occupying" it.
The amount of troops it takes to capture a map room is controlled by the "Map Room" setting in Object Settings. Each troop actually inflicts damage to the map room like a bomb, so it is controlled by the same values that control bomb damage to the other objects.
The default setting is 0.0015 which requires 10 Troops to capture the Map Room.
The individual troop damages the map room with 0.00015 damage
To change the troops, multiply how many troops you will require to capture by the individual troop damage amount.
For example:
Default Arena Setting 10 x 0.00015 = 0.0015
Modified Arena Setting 25 x 0.00015 = 0.00375
Add the 0.00375 to the Map Room setting and your fields now need 25 Troops to enter successfully in order to capture a field.