Like with any aircraft - the max endurance is had at Vbg - best glide. If such is not published, it's pretty close to Vy, normally. I don't know how this pertains to jets, but the fundamentals are the same. And what else is that max endurance is independent of altitude - you get no benefit for flying high, at least not with pistons. I would assume the same would be true for turbines as they're both aspirated.