The first endurance regen ability your Berserker will gain is Second Wind at level 72. Second Wind has an active time of 2 minutes and a reuse of 36 minutes.
Breather is the level 101 version, not exclusive to Berserker's by the way. It's claim to fame is a short reuse of 90s while having an active time of 42s.
The downside is, your Berserker can't gain endurance above a certain percentage ( 21% for rk1, 25% for rk2, 29% for rk3 ) while Breather is running.
The idea if for the Berserker to use their endurance until they go under the threshold (for instance, down to 15% endurance if using rk1), then use Breather to regen back to cap.
It is unlikely you can use the remaining 21% endurance before Breather is up again, therefor it is "infinite" endurance.
TLDR: Breather is awesome. Enjoy the constant endurance