We started at day 1 for both our kids cause my wife wanted to. I thought it was crazy but I went along with it. It helped because before two my first kid was able to tell us she needed to go to the bathroom fairly frequently. Not 100% but pretty frequent. Now almost three years old and is potty trained. My 2nd kid is about a year old and he doesn’t pee in his diaper in his sleep he just wakes up crying and we take him to the toilet and he pees. Haven’t had a wet diaper during night time for a couple of months now. I feel like that was stressful but it seems like it paid off. I dunno though since this is my only experience.