Figuring out the perfect waketime length is tricky. If you read through my newborn summaries with McKenna, you will see that her waketime length changed and changed often. So not only are you trying to figure out what is perfect, but you are trying to figure out where a target is that moves from day to day.
Where do you start?
Start with what is "normal" for "most" babies. There is a reason most babies fit into a certain time frame--it is best for most of them. That means there is an excellent chance that is best for your baby. Page 130 of On Becoming Babywise suggests 1-1.5 hours of waketime for your newborn. If you see my post on Optimal Waketime Length, you will see that 55% said 30-50 minutes was optimal for a 0-4 week old.
I found optimal for a newborn to start out well below an hour. I shoot for 30-50 minutes. I then extend it as needed. As the baby gets older, waketime length will get longer.
Also, not every wake time length of the day will be the exact same. Morning (the first of the day) is typically your shortest once baby is all settled in. Some babies will have the same length all day long, others will vary.
When you are working to figure out waketime length, start by focusing on the first one of the day. Once you have that one perfected, move on to the second one. Start that one with the same length as your first, then adjust if needed. Writing this information down is very helpful to keep track of it all. I have a Google Document of my Optimal Waketime Log that I used to see it all.
How Do You Evaluate?
Typically, if your newborn has a hard time falling asleep for a nap, she was up too long. If she sleeps but wakes early from her nap (and it isn't because of a growth spurt or some other reason), then it is also usually that the newborn was awake too long. In most cases, if a newborn does not nap well, the newborn was up too long.
As your baby gets older, if she takes a long time to fall asleep but isn't crying about it, then she was likely not up long enough.
Hitting A Moving Target
Once you nail waketime down, it will soon change. The good news is that the change is adding time. So at least once you have it down, you know to go up. I add only five minutes at a time when I add until we get it right.
Many brand newborns will not have a waketime every single waketime of the day. They will eat, then go right to sleep. That is okay. See Adding Waketime to Your Newborn's Day :http://babywisemom.blogspot.com/2008/11/adding-waketime-to-your-newborns-day.html.
Sometimes your baby will not sleep well because of overstimulation rather than simply needing a longer waketime length. See Overstimulation :http://babywisemom.blogspot.com/2008/12/overstimulation.html and Playtime: Don't Over Stimulate :http://babywisemom.blogspot.com/2008/07/playtime-dont-over-stimulate.html.
To sum up, as you work to figure out optimal waketime length for your newborn, keep in mind that many do not stay awake after a feeding for every feeding of the day, and that is okay. Start by having a 30-50 minute waketime. As you add time, add only five minutes at a time.
For more ideas on this, see Waketime: Length, Extending, and Calculating:http://babywisemom.blogspot.com/2008/02/waketime-length-extending-and.html.