It's a predetermined weight of retardant (Halon, incidentally, is no longer produced), and fires in a single shot. If the first bottle doesn't do the job, a second can be discharged.
Many of the 747's I fly in freight service do not have a lower cargo fire prevention system. They are equipped with detection, only.
That only applies to the B742. All B744's and B744F's have lower cargo fire suppression. There are four bottles. After FWD or AFT has been selected, the DISC switch is pressed. This discharges two bottles into the selected hold. The remaining two bottles then commence a metered discharge providing 195 minutes of fire suppressionon the B744 and 210 minutes on the B744F. The difference is because the B744 leaves two packs operating whereas the B744F reduces to one. The two metered bottled fully discharge on touchdown.
As had been said several times on this thread, in the case of a main deck fire on a B744F there is no fire suppression. You have a maximum of 15 minutes to get on the ground. If you can't achieve that then seriously consider putting it on any available stretch of water (while still under control).
Dave