SPB - i force weaned ds when I was pg, as it was too painful. When dd was born and he saw her nursing, he asked to try again, and I let him because I was fairly sure he wouldn't have his latch. Doh! He nursed more for a while, when I was too knackered to limit it and deal with the tantrums, but limited it to sleepytime (before bed and nap, and first thing in the morning, and sometimes after nap if we don't have to be anywhere) when dd was about 8 weeks old.
He loves nursing with dd, and will often ask to nurse together, and I allow it for as long as possible as I'm told it can help strengthen the sibling relationship. I get very touched out very quickly with him though, and can only nurse him for 10 minutes or so at a time, which is much less than he would like!