Helium does go through metal, although even in this case the amount is probably insignificant.

Helium also goes through whatever the sealing gasket is made from, at a much higher rate. http://lpc1.clpccd.cc.ca.us/lpc/tswain/permeation.pdf is a neat chart, probably the gasket is buna-n because it's cheap. I think that chart and https://en.wikipedia.org/wiki/Permeation are enough to get an order of magnitude answer to OP's question, but I'm not quite up to doing the math.

Serious vacuum systems have to worry about this sort of thing more than one might expect. I once had a project where two test engineers and myself spent around a week hunting for leaks in a vacuum system that turned out to be caused by my specifying Silicone o-rings instead of Viton.