The answer: so long as it is actively fermenting, not so much.
Once it is done fermenting, however...that's a different story, and you should rack your mead into container(s) small enough that you don't have much surface area exposed...basically you want it in a container such that it is filled all the way up to the neck, to shrink the surface area exposed to gas exchange.
As to how long to secondary...well generally speaking people age their meads for months. I know some people that go a year or a year and a half before bottling. My own personal policy is to stick it in a corner andn forget about it for a long, long time. Eventually, I'll be cleaning up or doing something else and I'll run across that carboy and think, "gee I wonder how long that's been sitting there now?" Then I will go look up my notes and figure out if it's time to do something with it. Apart from that, I make sure the airlock is filled up whenever I run across it.