If the box contains 305m of cable, can I make a 305m Ethernet run?

No. The box quantity tells you how much cable you bought; it does not rewrite the Ethernet cabling standards.

For conventional balanced-copper structured cabling, the familiar design limit is up to 90m for the permanent link and 100m for the complete channel including patch cords. A 305m box is enough raw cable for several runs, not one heroic run from one end of a building to another.