I don't follow a set 'routine' at all, however, DS2 who is 23 weeks old, gets tired about 2 to 2.30 hours after he has woken up. It's a great way to go about it - if he wakes up at 7, I can guarantee that he will be tired at 9 - 9.30. He sleeps for as long as he needs, and when he wakes up I start the clock again - he will be needing a sleep 2 to 2.30 hours later.
It often means that 'bedtime' is at different time, anytime between 7 and 9 really, but we stick to a 'routine' for that - bath, story, cuddle, bed. I suppose that as he gets older he will be able to stay awake for longer periods of time.
One of the books I read suggested that even babies who don't appear to have a natural routine do have one, and part of the solution is to write down their feed time and sleep time down for a few days and see if there's a patern. And then you can reinforce the patern and things become more predictable.
I have to admit that I enjoy the 'predictability' of my son's sleep, I know when to get home, go for a walk, go to the shops, etc without having a screaming baby who is overtired.
As for the feeds, what you are experience is fairly standard, DS2 eats when he is hungry and he wants it NOW. I just give him as much as he wants (or as little as he wants) whenever he wants it. It's pretty different every day. Like Honoria, we do give a 'dream feed' at about 10.30 pm, but sometimes he will only take 2 oz, sometimes 6 oz. He decides, we don't ever force feed him.