The one that lasted longest was the one by Gerry Conway, and that was about 15-16 issues. I think that one lasted as long as it did because it was smart enough to put the focus on the heroes trying to stop Carnage, such as they were. Kind of reminds me of how horror movie franchises will be titled after the villain/monster, but the focus in each installment is on those trying to survive against or stop the villain/monster.