I have just gone back to work, ds was 7m when I started back. He eats everything in sight and has dropped a lot of feeds, so only needed 2 4 or 5oz bottles a day when he started, and has now dropped to 1. I know some babies don?t drop feeds as quickly though, but there is a calculator on Kellymom about how much to leave, I think its 1oz per hour plus an extra one iirc.. I am expressing twice a day, and getting between 8 and 10 oz a day, which is plenty for my needs. I bought a really good pump in preparation, an Ameda Lacteline, and it?s so much better than the cheapo Tommee Tippee electrical one I had before, I got it for £50 second hand on ebay but given how much better it is would happily pay retail now! I built up a freezer stash before I went as well in case of emergencies, so that?s one thing I would definitely recommend.
My HR dept have been brilliant, they have booked a meeting room for me each day for half an hour (although I take nothing like that but it gives me a bit of flexibility in case I?m busy) and I just go and shut myself in and get on with it. On days that the room is needed for other things they have even moved out of their office and let me use that instead. I really can?t fault them, they didn?t bat an eyelid when I requested it, but I don?t know if anyone?s requested it before, I can?t think of the last person here who had a baby so I may well be the first. Make sure you take something with you when you express as you get much more when you are relaxed. I have a little video and pics of DS on my phone which I look at to start off, and then when the letdown comes I read emails/text etc. It?s amazing what a difference it makes.
I found the first few days I needed loads of sets of breast pads as I was not only adjusting to not feeding as much, but also I kept getting random letdowns whenever I thought about DS and was missing him, so take a few spare ones to avoid embarrassment! I spent a really warm day walking round with my jacket on as I?d leaked through to my shirt.
Also, for the first month, DS refused milk from the minder. It wasn?t a bottle issue as he happily took 2 bottles from his dad the day before I started back, we can only think it was because he didn?t feel comfortable enough with her ? he started taking it when he?d been there a month. It may not happen to you but just something to watch out for. We weren?t that worried as we knew he wasn?t starving as he was eating loads of solids and drinking water. And to be fair, he has plenty of fat reserves 
Sorry that was a bit of an essay!