I feel that it's way, WAY too short. Often times I won't even be able to get one off if I'm typing manually (yes, I'm a crazy who still does this sometimes), and I type REALLY fast. I get that the COMBO command is supposed to be the answer to this...but I don't think it solves the problem completely.
If you've played IRE combat before, then you'll know that who has the advantage in a fight can change VERY quickly, and being unable to adapt on the fly because the rage window is so short feels limiting. For example, say that you use a COMBO attack depending on the rage attack to take advantage of an ailment, and then your opponent cures that ailment part-way through; wouldn't you prefer to switch the rage attack to something more useful? I would. But you currently can't because the window is so short.
I would much rather see the rage attack window simply removed. Allow one rage attack when you enter a stance, and leave the window open for as long as you normally sit in a stance.