I've seen three different opinions on this. The first is, pump the number of strokes specified in the owners handbook (something like 3-4 strokes I think, I can't remember). The second is to pump grease until you see it start to ooze out of the seals. The third is to keep pumping until the grease coming out of the seals is new, fresh, clean grease - in other words until all the old grease has oozed out of the seals (this was from the MG-Owners Club in the UK).
I find the book recommendation is enough so just a little grease oozes out, and that's what I've usually stayed with.