First of all, congrats on your lease!
1. How often should I got to the stable to work with her?
If possible, every day! (Of course that isn't always realistic, so just do the best you can.) Even if it's just for 5 minutes.
We can even relate this to your riding lessons. Would you learn better by having 4 hours of lessons once every 2 months? Or by 30 minutes once a week, like you have been doing? Obviously, short and frequent is much better learning than long and infrequent.
2. How long should I spend working with her?
Time really isn't so much the key, as your progress and her behavior.
For example: I have been doing a lot of ground work with my yearling colt. One day, I was going to load him onto a slant load horse trailer with rear tack, for the first time. (He'd been hauled on stock trailers before, but not this type.) My goal for the session was to at least get both of his front feet into the trailer. And that goal could take me 20 seconds, or it could take 2 hours. Time isn't the issue. The training is.
He's been a great colt and I actually was able to completely load him onto the trailer (I didn't not close the butt bar on that training day). Load him twice, actually. And back off calmly one foot at a time. It took a total of maybe 2 minutes, if that.
So I quit for the day! After 2 minutes. I had exceeded the goal I set, and he was a good boy and learned something. Great. We're done.
So when you go to do your ground work, pick 1 or 2 things you want to work on (and no more than that). When you consistently get the correct response, great! Quit. No matter how much or how little time it took.
3. What should I be working on with her?
If are you comfortable with lunging, you can do that for a few minutes a day. The point is NOT to make her go round and round and get tired. Rather, the point is to have her paying attention to you. So ask her to change directions frequently, change gaits often, and stop/go a lot. Make her constantly be focused on you to be looking for her next cue. Since this can be intense for a horse, that's why I say a few minutes of lunging is usually fine.
You can also just work on leading her around the stable/barn. Focus on making her stay right by your shoulder. If she crowds you, put your hand up in the air to move her over, or (with the exess rope you are holding in your left hand) swing the excess rope behind you to smack her and move her over. If she gets ahead of you, give her one jerk on the leadrope to say "hey! I"m here."
It's all about being consistent.
And of course, spend plenty of time grooming her and getting to know her. Grooming is also great to know every detail on her body so you know if something is wrong (bump on her leg, a cut on her belly, etc) that could be a health problem.
when I do eventually ride her alone, how often/long should I do this. Does it make a difference if we ride in the arena vs outside, vs trail?
Depends on the horse and depends on you!
My old horse Beau, I usually rode him about 4 miles and I tried to ride almost every day. The mileage was for conditioning, as I did barrel racing with him, and he needed to be in great physical shape.
When I have young horses in training (age 2 to 4), I usually keep the riding light and just a couple of miles. For the first few rides under saddle, I may only ride for 15 minutes in a round pen, or even shorter. Depends how they are behaving.
With my current 6-yr-old who is the Energizer Bunny, he gets ridden a very minimum of 4 miles almost every day, usually 5 or 6 miles, but 2-3 miles of galloping. He's very energetic (also an easy keeper) and he needs the miles and the work.
That said, most of my riding is out on the open trails. I think that is the best type of riding you can do the majority of the time. Because even for myself, **I** get bored riding in an arena. So if I do any arena riding, it's probably going to be only about 30-40 minutes tops. I know if I am bored, I can't imagine what my horse feels like.
And then there are some people who like to ride their horse on the weekend only and only walk their horse a mile or two.
There's not a right or wrong. Depends on what you want to accomplish and depends on your horse.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.