So just to update this thread with what I know now...
My filly is being worked multiple times (short sessions) daily.
I got her there on March 8th, and I know that he started feeling her out (ground manners, etc.) that weekend, but didn't do anything other than that.
On March 13th, he was ground driving her (or long lining, whatever you like to call it), and was impressed that she automatically knew her directions. In his words, "I usually don't even try to turn them at first, since it just confuses them. But I did it just to see how she'd respond, and she performed it correctly." She's done it correctly ever since, too.
He even thought I was lying when I said that I hadn't worked with her on responding to bit pressure yet! Must be all that groundwork and leading we've done.
Sometime around March 16th he hooked her up to a tire, and said she didn't seem to care whatsoever. He had her drag it around the barn first (as he doesn't have an indoor arena, and that's his only enclosed area). Then they went outside.
Fast forward to March 20th (this last Wednesday), and I headed out to his place to see her drag the tire. Imagine my surprise when, upon arriving, I see her and a tall Percheron gelding outside dragging a training sled for the first time! Mind you, the gelding was doing most of the pulling, but she was definitely relaxed, and taking everything in stride. The gelding wanted to go, go, GO! But her attitude was more like, "Nah. I think I like walking better." Lol! I asked the trainer if she reacted when hooked up to the sled, and he said, "No, not really. Took everything in stride."
I think that the trainer is pretty impressed with her manners and willingness. I told him to go slow, as I didn't want to rush this and have her blow up, and he confirmed that he is, but she is pretty comfortable with anything. He said he might even have her (and the gelding) drive a cart this weekend.
Needless to say, I'm excited!
I'm so surprised at how well she's taking all of this! She's developed good manners by being worked with daily, but I never knew that she'd progress this fast! I'm going to make sure that she's comfortable with everything, before taking her home. I want a bomb-proof driving horse. That way, I can go out on my own, and not have a problem with her.
Also, I was wrong about him saying that he usually only takes a month to train them. He said that it usually takes him about 2 months to train one to drive.
I have seen him work with her, and he's very gentle with them. His approach seems to be, "Work them until they consistently get it right." He actually doesn't talk much when he's working her (or any horse, for that matter), and I've never seen him lift a finger towards my horse or towards any of his own. He's pretty quiet, really, which is something I really like about his attitude.