I found with mine that they kind of fall into a routine and you reinforce it.
I know that as a rule, newborns can't usually stay awake for longer than an hour or so, so after that time, I'd look for signs and put them down. Both mine fed on demand, so that was never an issue.
Bedtime was still the same as us at 3 weeks, but don't panic, and it should naturally work itself back. Dd is almost 5 months now and her/our routine is as follows:
7-9 (depending on toddler volume) wake up. Have a feed.
10-11 eye rub= back to bed for a bit.
12 or earlier wakes up for feed/play
between 1-3 back to bed for a bit
more feeds.
7 feed and bed
7-9 up and down the sodding stairs
1-4 wakes for a feed
3-6 wakes for a feed
So see, we do have a routine, but it varies and has to accomodate things like going shopping, taking ds to nursery, going for a walk etc.
BEsides, their routines change all the time. We worried a lot with Ds that we wouldn't be able to get him into a routine - especially in the early weeks when it's chaos - but it happens. They're quite often creatures of habit. Mind you, so am I!