I'll repeat what I've been told- horses attention spans are rather small, and it's best, when teaching something or training, to limit the real work to about a half hour. If it's a little over, that's okay. So it's up to you. Are you going to be really working during your ride? If you are, then save it for another day. If you just want to have a relaxing ride, then I don't see much of a problem.
If your mare starts getting frustrated before your half hour is up, try to end on a good note, then end early. Once a horse gets upset and frustrated, it's difficult to get them to refocus. I don't mean quit the second she's flipping out and still being disrespectful, but if she's minding you but seems a little frustrated, praise and end.
Does that make sense? Sorry for the novella. :P