If you're in armour mode it should only take 2 hits...if youre not then it can take 3. Pretty much armour mode it & you should win most 1 on 1's. Ive mainly been playing on xbox though & there seems to be a lot more drag towards the enemy player when punching.
It takes 3 hits if you have no energy, I believe. It still is 2 if you have energy and are not in armor, I THINK!
I thought of something else. If you have a secondary weapon out, you melee faster. So if you have time to pull out your Nova, for instance, you will hit faster.
On the flip side, if you're using a big ol' DSG or something that's great at hipfiring (a grendel) if you fire FIRST and then melee, it goes in rapid succession and you can almost insta-gib someone, provided you actually land your hipfire (with a DSG you practically have to put the barrel in his mouth).