Hi E.,
my daughter is 9 months old, and we started transitioning to solids when she was around 5 months. I'm still breastfeeding. My supply is definately less than it was a few months ago, but still enough to give her a few feedings a day (morning, evening, and middle of the night). I went back to work FT when she was 7 months, so I've been pumping at work, and that keeps up the supply. As your baby progresses to more and more solids, the breastmilk will become more of a secondary source of food. As long as you keep breastfeeding and pumping, your supply will stay up (again it will reduce over time, but that's OK, as your daughter's intake of solids will increase).
I think you can definately keep it up till 12 months. I didn't think I could do it, but once you get used to pumping it's not so bad. I actually have to travel a lot for work, and I've learned to pump on planes, in airports, in the office, etc. as i don't want to loose my supply. and it's going OK so far. My thinking is that even if I can give her a little bit of breastmilk it's a lot better than nothing. And mostl i want to make sure has a stash in the freezer for when i'm gone. So, don't stress yourself about it, keep breastfeeding when you can, and pumping the rest of the time, and you will be just fine!