On average 6 hours minimum, and planning a good battle arc , 2 or 3 months
But my case is a bit particular, English is not my native language.
I am also writing a novel with a fairly heavy Lore, an important background for the characters, their relationships as a close-knit family despite their differences, as well as the same level of detail for their enemies.
and to top it off, it is also a pretty heavy military plot. There are several factions with backgrounds and motives. Every major battle is somewhere in the real world, with place names, important objectives to complete and their objectives, strategy discussions, unexpected things, and trying to be as realistic as possible (during the battles, at least). Things are so complicated that I usually like to accompany it with a map to better explain where everything is and where the teams are moving, so the reader doesn't get lost.
For example, this map is of a fictitious battle in New York that took me about 2 months to plan. The Empire (the enemies, red) launched an attack on the UN headquarters and the U.S. Army and the National Guard are trying to repel the attack. But they can't do it without air support, the problem is that the Empire has under control the central air defense center in new york and also has under control the power plants that feed them. So the Alliance (the good guys, Blue) must first take over the power plants, get them running, then launch an attack on the anti-aircraft defense headquarters in the Empire State Building, and finally after all that launch a real attack and rescue the people trapped in the UN building.
And well, all that required time