The damage per shot listed in a siege weapon's stats is against other ships. Their anti-PoB damage output, last I asked, goes like this:
As for the damage output of a single siege ship, the two numbers that determine it are the ship's power regen and the weapon's efficiency. Taking the Battleship Siege Turret as an example, it does 10 000 damage for 400 000 power, giving it an efficiency of 0.025. To get the damage-per-second a ship can dish out over a long period of time, you simply multiply the power regen by the weapon's efficiency.
For example, the Legate has 151 000 u/s power regen. Multiplying it by the 0.025 efficiency of a Siege Turret gives us a value of 3 775 DPS. That is the maximum amount of damage a Legate will do to a PoB, regardless if it has one or four siege turrets.
I arrived to a similar conclusion to yours when I also assumed the stat card damage is what the weapons do to a PoB, but their actual damage output is much lower. Because of this, your result of 85 minutes is way off the mark, though far as I know the amount of shooting is still much less than it used to be.