Not all the coolant will drain. Probably 1/2 a gallon to an entire gallon will remain within the engine block (I'm not too familiar with the M119 but I know its predecessor the M117 holds a lot in the block). You really should remove both of the block drains if you're doing a coolant flush. Anything less is only flushing half the system!

If you have the owner's manual it will tell you within it. I'm going to guess about 3.5 to 4 gallons.