With mine I probably started around 5 or 6 weeks (before that I tended to have them with me in the moses basket or on my lap all evening rather then trying to put them to bed in their own room).
Obviously I don't mean leaving to cry or scream or anything but I'd feed, burp, cuddle and pat then put down in the moses basket awake. They'd usually cry or fuss and then I'd pop back (without necessarily rushing - so after a few mins maybe), pat a bit more or rock the moses basket then go out again and repeat ad infinitum until they dropped off. With DD2 I was much more relaxed (and anyway usually busy with reading DD1 a story or something) so possibly I left her a bit longer than I would have done with DD1. Within 10 days or so of this they both became very good self settlers.
Our little boy has sort of naturally found his own way in the last week or so (fingers crossed it continutes). He's 7 weeks old. He started to sleep from 8.15 ish after never sleeping before 10.30. We just kept an eye on when he wanted fed and his 7.30 feed would finish just before 8. We then decided to try and put him in his moses basket in the bedroom to see what would happen. He was quite happy lying there himself for about 10 minutes before drifting off to sleep. I'd never leave him crying though as I feel that is too early.
Mine were 12 weeks before I began to (gently) dictate bed time. They didn't seem ready before then. Then I would go to them as often as they needed me, but would not bring them back downstairs - it was bed time!
i started with DD at about 2 or 3 weeks, just letting her cry herself down. not fits of craziness or anything, more like discharging of batteries, once all else was ok (food , nappy etc etc).
worked well and night time sleep got longer and longer. now at 8.5 months she settles like a dream at night - 7.15/30 sharp and sleeps until 7am. it is marvellous and i'm glad we did this with her early, as she is MUCH LOUDER NOW
it helped to pick a bedtime around when she naturally got very tired and it took a few weeks to bed in, but once it did that was that. i love DD but guaranteed peaceful evenings are amazing, especially as she seems to nap less and less in the day now
We started doing a bedtime routine at around 2wks. We quickly discovered that dd never cries down - up, up, louder and louder! There was no way I was going to leave her to scream and what saved my life and sorted out her going to sleep easily and being able to leave her awake was a dummy. We began with replacing it so many times before she finally fell properly asleep but since about 6 wks, she usually goes straight off to sleep at 7pm once dummy is in. Sometimes, it still has to be replaced once.
I have a feeling though that dummy may end up backfiring on us and will have to be removed at some point but it has done the trick unto now without having to leave her to scream.