Try the Beginners Guide on the ivillage exclusively pumping forum. This is quite detailed and gives a lot of good advice about how often to pump and maintaining supply. It also gives a very useful brief survey of how much people were pumping and babies were eating. Its just an informal survey of forum members, but still quite useful because it details the wide variations.
forums.ivillage.com/t5/Roll-Calls-Introductions/Updated-Beginner-s-Guide-looking-for-input-and-any-additions/m-p/116687945/message-uid/116687945#U116687945
During the first few months they recommend pumping often to up your supply. I'm now cutting down the number of pumps I do gradually, maintaining the same supply over less pumps (down from 8 to 6). I don't produce a huge amount, but I produce more than I can get the baby to drink.
If your out you can just wait for a bit - depends how long your out for. The key thing for supply is the number of pumps a day and draining when you do it in the experience of the women posting on that forum. So just ensure you put that extra pump in. I don't find I get any drop in supply if I'm out and delay a pump that way. Done that for a gap of up to 6 hours so far.
I use a little bit of forumla, because I don't have such a large supply that I can afford to waste any EBM. My daughter has a habit of acting like she wants more milk, taking a mouthful, dribbling it out and then grinning like a loon while not drinking any more. I make up the EBM bottle I know she will drink and then offer her a bit of formula at the end for her to dribble. She'll drink the top up maybe once a day and at random times.
The hardest thing about pumping is that it takes time. Not always more time than BF (my husband does the last feed of the day while I pump for example), but time where its quite hard to be doing things with the baby (my daughter has discovered how to grab things which means pumping with her on my lap is a constant battle). The good thing is that your husband can get involved in the feeding too and thats a nice experience he won't have had as much if you BF all the girl.
I strongly recommend rigging your pump to use it hands free. I use the Medala Freestyle which is nice because its quite small and can be attached to a belt that comes with it so you can use it and move. I can't do a huge amount because the bottles get in the way but I can go into the kitchen and grab a drink. I sort of wedge the horns into my nursing bras and do them back up - not 100% secure but I've not had one fall yet. You can also make a pumping bra by making some holes for the horns in an old bra. If you're going to pump a lot at least you can then do something else at the same time.
I'm not sure how long I'll pump for. I was planning to BF until at least 6 months and then see. I've set myself the target of 6 months and then I'll see where I am. If I can get my current supply out in 4 pumps a day I'll go on for a bit longer, otherwise I'll stop then. I plan to be finished by the time I go back to work at the latest.