I have got an Ergo (but no baby yet ), and the instructions don't give any guidance on how long baby can be in sling for.
I remember that when I was researching some slings e.g. Baby Bjorn (I think), they had a warning not to have baby in sling for more than 2 hours at a time.
Is there any particular guidance on how long you can carry baby for? Will a newborn complain when it has had enough (I have the infant insert)? What is the harm caused by carrying baby for too long?