Everyone is different. There's no specific deadline. I figure when they bite, they're done. Most definitely done when they can ask for it with real words. I did daycare for a few kids when my boys were very small (cheaper than working and paying for daycare). One woman brought her four-year-old in for the interview. He sat on her lap during our conversation. Okay, no biggee. But then he announced "Want tit, mommy" and proceeded to unbutton her shirt and latch on. My only thought was "I hope he drinks from a cup, too"!