We used it with Isobel when she was newborn, we seemed to see some effect straight away but an even better effect once she'd been having it a week or two. I've heard a lot of people say it took about 2 weeks.
We could tell if we forgot to use it, but then at around 3 months old we forgot to take it on a night away and she was fine so I experimented without it and she no longer needed it then. It was brilliant when she did need it though!