I have had 2 c-sections. with the first my milk came in day 5. with the second I also had GD and my milk came in about day 3-4. The difference I feel was because DS was taken from me pretty much for the first 24hrs (only brought in for a few feeds) with DS2- because GD babies can have low BSL- I demanded that he be allowed to feed ASAP (he was feeding in recovery and continued for 1.5hrs) and he didn't leave me for the first 24hrs (I slept him on my stomach in the ward and allowed him to feed whenever).
It may have also helped that I started expressing milk from 34 weeks (just a few mls of colostrum each time) to have on hand in case DS2 needed top up feeds (common with GD babies) but he didn't need them because he fed so well.