It really depends on what product you're referring to. If a true deep conditioner, then probably not a good idea. And even though it might be okay with other conditioners, it's not really necessary. After a certain point/amount of time, a conditioner doesn't do anything else. HTH
i do it all the time, i'll slap on a deep conditioner at night, and rinse it out in the morning. no harm done. if you do it with a moisturizing conditioner, then it's best to perk up your protein, so your hair doesn't get mushy.
^^that's an excellent product. I think you could safely leave it on overnight, but your results would be just as wonderful if you left it in for 20-30 minutes w/ heat or an hour without heat.
I have read where some have left conditioner in for over 24 hours. Seems like overkill to me. But if they persist there must be some benefit.
Probably depends on your hair texture and whether you are chemically treated or natural.