I didnt start dfing until about 5 weeks as I was breastfeeding, but once started my ds was flying. It did take my little man a week or so before he started to get the benefits of the df. I would say the earlier you start a routine the better. We would always go to bed about 11.30pm so I started the df at about 11pm, so ds would get his nightime feed at about 7.30pm df at 11pm and sleep until 8am..... I know people who didnt df and at 7months their babies arent sleep through the night.
Best of luck and enjoy your time with you baby as they grow so quick (my ds is 7 months already)
Taylor