I'll just use my experience with DS and you can form your own opinion

I used a dummy with DS from about 4 or 5 weeks old I think. (was a while ago!) It did not in any way affect my milk supply. I fed DS till he was 15.5 months old! I only used to use his Dummy for sleep times. He (as far as I can tell) knew the difference between the sweet sweet boobie and the plastic of the avent dummy (I was dumb and went for the 'cute' brand...and one of the most expensive). Like Barb said, I never used the dummy to space out feeds. I knew what he needed when he needed it, and if it was a feed, then I'd give it to him. Of course this kept my milk supply where it needed to be....just perfect for him.

As far as lenths of feeds, I had this all written down (in my desperation to help work DS rhythm out) and he used to feed for 10 - 15 mins each boob. When he was REALLY REALLY tiny, he only used to have one boob, but then I chucked both of them in and that worked well. I considered him to be a really efficient feeder, he knew what he was doing, and my boobs knew too!!!

If you think the dummy will help, then try it. That's what it's all about as far as i'm concerned. Working it out, and once you find out what works, keep going

Good luck