Yes, until the battle concludes.

yes - frenzy bonus lasts whole battle, no matter how long it will be and is cumulative...

if you have frenzy 3:

your bone dragons kill 4 stacks: 4x6 att = +24 attack... till the bitter end :)

In general Frenzy was the skill for my vampires which I sent always in the front lines and longer the battle lasted, easier it was for me...

I believe the Frenzy skill is especially invaluable to those pursuing single stack strategy.

@Gatts

Not that it makes much sense, but do you still get the bonus if you kill friendly stacks like Thorns or Undead raised by Necromancer/Necro Call?

You probably wanted to spell it " especially valuable"... as I used single stack strategy - vamps + gloth's armor + armie (understand : my familiar call of aramageddon) and as armageddon weakened all enemy stacks but did not kill them - my vamps just "harvested" them - where frezny was my main motivator to go for the weakest first (normally in HOMM I went first for the strongest :) what a change in my strategy only due one skill... great job devs), so my vamps harvested not only all blood, but also all ATT frenzy bonuses

I actually hurt my own stacks when they gone frenzy (ghosts) but well... I never killed hand-to-hand or with direct range attack my own stack so actually I totally don't know... but good question!

His use is actually fine. Valuable and invaluable pretty much means the same thing (like "flammable" and "inflammable.")