I stopped when she was three for a few reasons but the biggest one was because I was traumatized about a month after she was born and had a hard time giving it up.
She was born two months premature and I had a scare about a month after we got her home. I fed her and put her in her crib and passed out in the bed next to her. About 10 minutes later, just as I was about to fall off the edge into sleep I heard a little noise coming from her. I couldn't place it and it was so soft and unassuming that I almost closed my eyes and let myself plummet into sleep. But something made me check, just in case, and it was a good thing I did. My little girl was lying on her back and had vomited. It had filled her mouth but she wasn't turning her head to clear it. It had filled up her mouth and spilled out the sides to dribble down both cheeks. She was choking on her own puke. She wasn't turning her head!
I snatched her up and flipped her over, like upturning a little vomit filled tea cup, so she could expel her last meal but it shook me to my core. It took me a long time to move her into her own room and even before that I had trouble sleeping. I kept startling awake shortly after falling asleep to check and make sure she wasn't choking.
I had a hard time letting go of the monitor because of that incident. I used to crank it loud so I could hear her slight movements and even her breathing. It hummed something fierce and my husband was a little annoyed but over time, I turned the volume down and eventually put it away.
Now she wakes up, runs in my room and jumps on me yelling, "GOOD MORNING MOMMY!" and I always mumble, "What on earth happened to my quiet baby?!?" to which she'll reply, "I DON'T KNOW!"
Me neither kiddo! Oy vey!
So I guess the short answer would be, when you feel comfortable giving it up.