“Do I wait until 90 minutes are up to start trying to get my baby to sleep? Or should I start trying to soothe her before the 90 minutes are up so that she'll fall asleep at about 90 minutes? What sort of window of time do I have before another wakeful period starts? 5 minutes? 10? 1/2 an hour?”
- Pamela - sleep deprived parent. :)

Excellent question. Here's my advice.

#1. I wouldn't wait until the 90 minutes are up to start trying to get the baby to sleep. Your baby's signs of sleepiness should be your guide. When you see the signs (or hear the signs, as the case may be!), that's your cue that the sleepiness window is "open" and that you should try to respond as soon as you reasonably can.

Which leads us to . . .

#2. How long is the window open, you ask? I only wish I knew the exact answer to this question. It seems to me that babies are not all the same, that this may vary from one baby to the next. Some babies seem to have a 5-minute window, others may have 10 or 15 minutes. Your own experiences will be a more useful guide for you than any number I would come up with.