There's really no set time, but I would say when the baby starts sleeping through the night, and there are no more night feeds.
If you are a deep sleeper, or in a far room; if you're afraid you'll miss them crying
, longer. If you keep it, turn the volume down, so you can get a good night sleep
, low enough to only hear crying. Some moms keep it till kids are in a big bed, and come and get them.