I started using the routine as a guide, from about 9 or 10 weeks, and it helped a lot to establish some order to my and my baby's day, which made our family life easier. But I would add a number of caveats and reservations to following GF's advice:
1. As others mentioned, I don't think the GF routine is advisable in the early weeks. In my case, my newborn fed much more often than advised by the routine (especially during growth spurts), and personally, I don't think it would have been good for establishing our breastfeeding relationship.
2. My baby did not sleep through the night until several months later, so the main benefit of using the routine was in making our daytimes easier. My bub gradually reduced the number of his night feeds down to one night feed by around 12 weeks which was quite manageable, but this process of reduction started before I began using the GF routine, and I attribute it more to the techniques in the No-Cry Sleep Solution book. GF suggests leaving them to cry in their cot for some time virtually from birth, which I couldn't do.
3. I did not take the routine as set in stone. e.g. my baby would sometimes get hungry well before the scheduled feed times, and I would simply feed him earlier. The detailed nature of the GF routine implicitly puts you under pressure to adhere to all the times quite strictly, but this was not always a humane option!
4. Your baby may find his/her own routine after a few weeks, rather than you having to impose an artificial one from a book. Although mine cried a lot less as time went on, his feeds and daytime sleeps continued to be quite unpredictable and due to the general chaos this created, I felt it worth giving GF a try.
5. I founded the GF book itself quite painful reading as she came across as heartless and bossy!
Despite these caveats, however, I’m glad I used the GF routine, if only as a guide. It definitely helped me decipher my baby's cries and needs. Before using the routine, I was often unsure whether he was fussing out of hunger, sleepiness, over-stimulation, needing a nappy change, etc. But after starting the GF routine, I could usually calm him simply by doing what the routine advised at around that time of day (e.g. feed, nap, outdoor walk), often to my surprise! I do believe he became a more contented and easier baby with the routine. It took a lot of the guesswork out of first-time mothering and gave me more confidence in handling the baby, compared to when he was feeding and sleeping ‘on demand’. Plus, I could plan daily activities and outings e.g. my husband and I could enjoy a quiet dinner together in the evenings, after putting the little one to bed at 7pm. Sorry for long post, but hope this is helpful!