Not as much as you might think, I empty it maybe once a year. Probably a couple of tablespoons each time after 8-10kmls.
I have had the manifold off a couple of times since fitting the trap and oil no longer runs out like it did before, so it seems to do the job.
I have a few manifolds stripped down to investigate the cause of these failures and attempt to engineer a better job.
But so far have not come up with a cost viable solution.