How long is too long to let my baby cry himself to sleep?

I tend to lean more towards attachment parenting practices, but my poor son won't go to sleep unless he is nursing. He won't take a pacifier nor does he suck his thumb or fingers, and its gotten so bad that if my falls out of his mouth at night he wakes up and starts crying right away. I could tell today he needed a nap, but I just can't sit down and nurse for a few hours, so I decided to try just laying him in his crib and seeing if he would cry himself to sleep. He's been crying for about half an hour now, and its killing me to hear him cry. How much longer should I let him cry? Does anyone have any other suggestions for me?