You don't. You observe them and learn their natural pattern and sleep needs. For example, when my DS was around 4 months old, I knew that he could only be awake for about 2 hours at a time. Any longer than that and he started getting cranky. So, 2 hours after he woke up from his last nap, I would nurse him and cuddle him until he fell asleep again. After a while, I noticed he wasn't falling asleep as easily, so I started waiting a while longer to put him down for his naps. Eventually, he got to where he was taking two regular(ish) naps a day. Now he's transitioning into one nap a day. I noticed that he was having trouble falling asleep at his morning nap time and sleeping for only a short amount of time. Then he wouldn't be tired again until about 4 in the afternoon. So, I'm keeping him up until after lunch and letting him take a good long nap. Just watch your baby's cues.
at 7:48 AM on Jan. 31, 2010