I'm not quite sure what you mean by undershot but I guess that one is longer than it should be or it wouldn't have been trimmed.
I have a slightly crossed beak pullet with a longer bottom beak than top. She couldn't drink properly so I had the bottom beak trimmed and it had to go just into the area that bled. This beak has not regrown at all.
The top beak, just to be difficult, then decided when the bottom wasn't stopping it anymore to grow a real hook on the end. This I have to trim regularly so she can eat properly. It is only growing off the tip of her top beak so never gets trimmed near far enough that it would bleed. It regrown quite quickly.
So to answer your question I think it will depend how much was cut off and if they had to go back into the bleeding area.