Warhammer 40,000: Inquisitor - Martyr

I counted it out to be around 20 seconds. If you want, you can grind it out in places that have poison gas. Take damage for 20 seconds until you get the Unbreakable bonus, leave area for a few seconds, repeat.

One of the easiest ways to farm this out later in the main campaign there are mech missions. During these mech missions just throw yourself into a group of tons of enemies and you will eventually get the bonus. There are 2-3 mech missions so if you dont have it by the last one you can restart the mission to get it.