LO has been on formula for about 3 weeks now. She is 14 weeks old. Before FFing I fed on demand, and still do. She has settled herself in a routine. She takes 5 to 6 bottles per day every 3 hours, 5oz per feed. If she wants more, she gets more.
As for mixing formula, I use the powder and mix it as we go. I use "nursery" water and mix it when she is hungry. She takes her formula at room temperature. If I ever am out of nursery water, then I would boil some, let it cool, and make feeds as I go. I haven't been out of water yet, so I haven't done that. When my son was born 10 years ago, we used liquid and I mixed it and made bottles in advance. I have found the powder is quite a bit cheaper though.