I drained my pan at 105K or so and the fluid looked horrible. Probably had never been touched. I put 3.5 quarts back in and got the trans. warm and when I pulled the check plug fluid gushed out. So apparently 2-3 quarts is all the pan holds.
My thinking was that I'll do this every 6 months until the fluid coming out starts looking red again. But now I'm having second thoughts. This fluid is $9/quart at my dealer, and it'll take numerous 3 quart flushes to get the old fluid all purged. A second grade student could figure out that it's going to get expensive fast. My dealer will hook it up to the machine and do it all in 10 minutes for $180.
I've always heard that flushing an old trans. is a bad idea. But I can't say that I know anybody personally who has had a trans. die immediately after getting a flush done. So who knows...