In my experience, it takes about 15 minutes to explain the game (goal, round structure, all the things a creature can do on its turn, skim through the spells in the opponent's spell book without going into detail regarding each keyword), then about 45 minutes to play through the Apprentice match. Then it takes 90-120 minutes to play the full 120-point match... or we stop when we run out of allotted time, but usually by this point they're already realizing just how awesome Mage Wars is and how many things they could do (or do differently).
Try not to bog things down with keywords. I generally end up playing the Priestess, and I summon a soldier with a defense and a Unicorn with Regenerate just to show what those things do, and then I play them aggressively to keep the match short. I try to avoid spells and abilities dealing Daze/Stun/Sleep, etc, and I don't heal much (aside from Regenerate) as I don't want to bog us down with rules/keyword explanations and I don't want to draw out the game.
If you don't have time for both at your event (because you want to run a lot of demos), then you'll still need about an hour per new player to teach and run a single Apprentice match. If you want to oversee two new players playing against each other, add 30 minutes and try to help them plan their spells without absolutely telling them what to do (because that isn't fun for them)... e.g. it might be useful to narrow their options to a few good choices (e.g. "if I were you, I might try this or that or that.").