RL, so sorry you have to say goodbye to your mare, although it sounds like it's the best, kindest thing for her. It's always so hard, but it really is a gift to end their suffering.
I've seen several horses put down and it's usually very fast -- they're given the drug and drop almost instantly. I do have to warn you, though, it isn't always like that. The vet always checks for heart rate & eye response and occasionally has to admiister a second dose. I've seen one equine euthanasia that was a bit disturbing (won't go into details) but overall it really wasn't too bad to witness. I guess it just depends on what an individual can handle. I personally feel that every horse deserves to have someone he/she knows with him/her at the end, and a few times I've stepped in to be the strong one when friends just couldn't do it. So, if you have the strength to accompany your mare, I'm sure she will appreciate it; if not, try to get someone else to stay with her.