I also changed a clutch slave cylinder this week. I had the line open
for about 10 minutes and a lot of fluid in the master emptied. With
everything back together, it took maybe 60-80 pedal strokes before we
got enough air out of the system to feel some pedal pressure. If I had
not done this before and knew how long it can take, I might have gotten
worried. My helper was worried, but I told him to keep pumping the
pedal.