I think every child is different. With my oldest, I didn't have a set amount of time in mind to breastfeed. However, when she refused to eat any type of baby food or cereal, it quickly became apparent that we were going to be breastfeeding until she could eat table food. She also would not take a bottle when I was gone. She was not a fussy baby at all, though, but then she would keep me up half the night eating. To wean her, I ended up leaving her with my parents for a weekend. That was difficult on all of us (and painful) since she had only been nursing still. That was when she was just over a year.
With my 2nd, I planned on going at least a year. However, I had to go back to work after 3 months, and the only place I could pump was a bathroom stall. The pump wasn't the best, either, but after 1 month, and me not being able to go when my milk would let down and only getting a couple of ounces during the day at work, we switched her to formula.
With my 3rd, I knew I was staying home after I had her so I planned on at least a year again. We made the mistake of never introducing a bottle to her so we could never go anywhere without her. I didn't make it to a year with her, either because she had other plans. One day, at around 9 months, she decided she wanted nothing to do with it, and she never looked back.
My 4th I am still nursing. My goal with her had been a year, and we hit that last Friday. She is really only nursing in the morning and at night. She eats table food and has sippy cups the rest of the day. I guess I am planning to go as long as she wants, but it needs to stop BEFORE the next one gets here because I don't think I'm cut out for tandem nursing. I have been back at work since she was 7 weeks, and there have been times where it has been difficult and many times when I could have given up. However, I stuck with it, and I am so glad I did. We did have to supplement with formula at times, but I felt that was better than switching over entirely.