I have found a lot of great articles and resources on nap times and waketimes. If you keep your baby awake too long, they will not nap as well, and will become overtired which makes for getting them to fall asleep a challenge. The best blog that I have found on the topic of sleep is My Baby Sleep Guide, which many of the articles in this post will be from. It is a GREAT resource for all types of sleep topics.
One of the most IMPORTANT things to remember is the Wake-Eat-Play cycle. Naps need to be followed by a nap, followed by waketime. Switching wake time and meals is a recipe for disaster and poor sleeping habits.
During the first 2 months, if your baby is not napping well, first try cutting down on waketime by 15 minute increments. Baby may be overtired and needing to go to sleep earlier. For newborns, waketime is usually only long enough to include a feeding and a diaper change before going back to sleep. Once the baby is between 3 - 5 months, the naps should be 1 1/2 - 2 hours long, 3 times a day. So waketime needs to be planned accordingly. The following averages are from My Baby Sleep Guide, the article also includes wake time averages from several other popular sleep training guides.
Newborn: 30-60 minutes
4 weeks: 45-60 minutes
1-2 months: 45-70 minutes
2-3 months: 50-80 minutes
3-4 months: 60-90 minutes
4-5 months: 75 mins.-2 hours
5-6 months: 75 mins- 2 hours 10 mins.
6-7 months: 95 mins- 2 hour 20 mins.
7-8 months: 1 hour 45 mins- 2 hours 30 mins.
8-9 months: 1 hour45 mins.-3 hours
9-10 months: 2-3 hours 15 mins.
10-11 months: 2 hours 15 min-3 hours 45 mins.
11-12 monhts: 2 hours 15 mins-4 hours
Boston is currently doing between 60-75 minutes of waketime. In the chart 4-5 months says 75 minutes - 2 hours, but I go by his adjusted age which is just over 3 months.
This Sleep/WT chart is also a good resource, but every baby is different and needs to be adjusted accordingly. Personally, I find the chart helpful, but I do not agree with the times in the "max total daily nap time" column. Sleep/WT chart
I have found two articles particularly helpful for babies who wake early from their naps. The first is Wake to Sleep. A baby has a sleep cycle of 45 minutes, which means often a baby will wake at the 45 minute mark while transitioning into the next cycle and have trouble falling back asleep. Wake to sleep involves gently stirring the baby at about the 30 minute mark (not until they are awake, just until they come out of the deep sleep) and letting them settle back in. They will in theory skip the transition and go right back into the next phase of sleep.
The second article is Pick Up / Put Down (PU/PD). For the PU/PD method, when a baby wakes early from the nap, you go in and pick them up (do not look at them or talk to them) and hold them gently to your chest until they calm down, then put them back down for their nap.
Nap lengths: what's average? This article (as well as Babywise) states that for a 2 month old, naps need to be 1 1/2 - 2 hours long to ensure adequate rest. In addition to the 3 day-time naps, there is usually an early evening "mini" nap which is not included, this may be as short as 20 minutes.