3.8 seconds is way too long, I watched her try to cast it on a Kagouti and it headbutts her breaking the cast and wasting the built up ultimate. This applies to a lot of mobs if she is the one they are focused on attacking. Heavy Attacks, bashes, forward leaps, zombie vomit attack, and of course headbutts are going to make her ultimate useless with how much time she spends building up before bringing it down. I think 2.0 to 2.5 seconds is more than long enough for a companion's ultimate cast time.