I keep my 17 week old up on purpose more during the day or else she is awake at night! Since about 13-14 weeks This is how it goes:
Wake 7.30am feed
Sleep 9am-11am
Wake 11am feed
Sleep 1pm-3pm
Feed 3pm
then generally another 45m to 1 hours sleep between 5 and 6
Feed 6pm
Sleep 7pm
It really does go like this but it has been a stretch keeping her up two hours at a time- the last 20 mins is usually me carrying or holding and doing stuff with one hand. If I only keep her up 1-1.5hrs she wakes around 9pm till about 12!! I have to wake her though after her 2 x 2hr sleeps or else she will sleep and sleep. She is hardly grumpy this way and when I put her down for sleep there is not a peep from her. I have read that 1.5hrs at this age (12w) is good up time. I got this off the Ngala (parenting place) website for a 3-6month (very general though)
Total sleep
sleep15 to 16 hours total5 hours during day in 2-3 blocks of sleep.10 hours during nightinterrupted by feeds.
Sleep patterns
2 to 3 day sleeps 1 to 2.5 hours each;10 hours sleep overnight – may feed 4 to 6 hourly.
Awake patterns
1.5 to 2.5 hours awake time between day sleeps;one period of 2 to 3 hours of awake time.
HTH