immediately they wake up, and if you've got a place where others have been even better. I have always slept with babies in my arms wrapped in a soft towel and then know if they move etc.
had the greatest difficulty with a 3yr old blind pug who had been in a cage for 23 1/2 hrs daily and only peed once daily- v unusual for boys who usually pee for England!! had to take him on lead as he didn't know where he was, a lng haul and eventually got it BUT then would come in and job on the inddian carpet----------he is perfect now.