#2
by Coolhand
Each exchange of fire ("The attacker fires ..." followed by "The defender fires..." bit) is a round. Combat lasts until one side is destroyed or until the end of six rounds of combat, at which point the remaining attack force (if any) withdraws back to its launch coordinates.
The number of ships and defenses firing will dwindle as casualties are removed. If you were both down to near-zero, sounds like a pretty even battle.
After a fight has ended, approximately 70% of the planetary defenses can be repaired; this happens instantly. So if I had 2 attacks 1 second apart, hitting the same planet, my second attack would face 70% of the defenses that the first attack faced - even if the first attack "destroyed" everything.
For this reason cargo-only follow-ups are only a good idea if the planet has little to no planetary defenses to begin with.
I got the rationale of a New York cop
I got the patience of a chopping block