How far in advance should I let mom who's living in an assisted living know that I am leaving on a vacation?

This may sound really stupid, but I'd consider making almost like an Advent calendar with when you're back, so Mom can mark off the days until you return. Or maybe something she can open each day that says what you'll be doing and where you'll be (if you're touring or something like that). So she can be a part of what you're doing. Just an idea.

And I'd wait until you're ready to go. No reason to prepare her, since it won't change anything for her and may just cause her to try to sabotage your leaving. Go and have a GREAT time!

I would plant the seed a few days before, then remind her a couple days before and then the last day...but only you can judge what's appropriate depending on if she will dwell on it and fret on it the whole time leading up to your departure. If she is going to guilt you or get sick or have a spell prior to your leaving, then by all means don't say anything until you are on your way to airport and then just say it will be a couple days and you'll be back in no time...likely her sense of time will hardly notice your absence.

Go and have a good time knowing she is in good hands.

This is what I'm going to do, I'm going away in Oct. (I can't freaking wait) my sister is coming to stay here with Mom, I will tell her the night before I leave and stress the fact that my Sis will be with her...If I tell her before that she'll constantly ask me when am I going ? She had panic attacks when I'm not here so I wait till last minute...

I absolutely agree let the facility have contact information but go in to see Mom on your way to the airport, tell her, say good bye and leave

Okay, Im new to this, but not new to caregiving. I envy you in a way because you have support .. the assisted living facility will care for your mother. So if she tries to sabotage your rest and vacation time, it would be my idea that you wait until last time you see her. Especially if she has any memory loss. Does that sound too cruel? Its just my gut reaction.