Forget months and years. Some parents live with their adult children for decades!
The idea of housing a white-haired version of your struggling son or daughter might seem ridiculous now, but it can -- and does -- happen. This is precisely why you should all agree to a move-out date before your kid moves back in. If the deadline passes and your grown child still isn't ready to move out, you can have a conversation about extending it.
In the beginning, it's important for you to establish that his or her time at home won't be forever. Regardless if it's a few months or years down the road, talk about the future before it's too uncomfortable to bring up the subject.