The most-quoted figure is 21 days, but that's a myth: it came from a 1960s observation about plastic-surgery patients adjusting to their new appearance and was later misapplied to habit formation.
The more robust data comes from Phillippa Lally and colleagues at University College London: it took participants an average of 66 days to make a behavior automatic, but the range stretched from 18 to 254 days depending on the individual and how demanding the habit was. Drinking a glass of water becomes automatic quickly; going for a daily run takes far longer.
The important takeaway: missing a single day doesn't meaningfully derail the process — the same study found that occasional missed days did no measurable damage to long-term automaticity. What actually matters is maintaining continuity, not counting days. That's why Zinciri Kırma focuses on sustainable consistency rather than a flawless streak: when you miss a day it lets you keep going with skip and repair credits instead of resetting the chain, because the path to 66 days runs through not quitting at the first slip.