I don't recall seeing anything codifying the topic. Generally given the nature of Shadowrun combats, if you're caught in one and you don't already have armor on about the least effective thing you can do is spending your valuable pass(es) putting armor on. Instead of doing things that render the need for armor moot (like killing the other guy first, or breaking contact, or etc).
Still, if it's really important.. you can rule of thumb it. Something like an armored jacket can probably be slipped on in one complex action, if it were very handily hanging somewhere nearby. Putting on any kind of bodysuit or spirits-forbid heavy armor is probably just flatly going to take too long to be worth attempting mid-combat.