Hmm - couldn't really give you any definite time frame. It depends on how much racing I do. I tend to change my bearings every 3 gallons of track time (or sooner if they feel notchy of course), but 3 gallons of actual racing is a lot of tanks.
To be honest, I haven't really noticed that much difference in wear personally. That starter box shock loading is something I have been told about, but I think that would also depend on how you have your starter box set up to the flywheel.
I have had a set of Answer ceramics in my MG for ages now, no problems at all.
They give a quicker spool up, and a little more performance all round. Slightly better fuel economy too, due to the lowered rolling resistance of the bearing.