You shouldn't spew too much out. There should be a bleeder valve you have to loosen and tighten to bleed it. Put the pedal to the floor, tighten the valve, then pull the pedal up. Open the valve and push the pedal down again. Keep doing that until it's shooting out fluid the whole time you press it down.

*NOTE* This is all for a Mazda hydraulic clutch system. It was 45 min for me to change a clutch master cyl too, the slave should only be like 15-30.