Heat limits the length and intensity of ship-to-ship combat. Starships generate enormous heat when they fire high-energy weapons, perform maneuvering burns, and run on-board combat electronics.
In combat, warships produce heat more quickly than they can disperse it. As heat builds within a vessel, the crewed spaces become increasingly uncomfortable. Before the heat reaches lethal levels, a ship must win or retreat by entering FTL. After an FTL run, the ships halts, shuts down non-essential systems, and activates the heat radiation gear.
Combat endurance varies by ship design and by the battle's location. Battles in the deep cold of interstellar space can go on for some time. Engagements close to a star are brief. Since habitable worlds are usually close to a star, battles over them are usually more frantic.