A behavior is automatic to the extent that it is fast, efficient, runs outside of awareness, and is hard to inhibit once started. The habit literature uses automaticity, not frequency, as the marker of whether a behavior has crossed into habit territory. You can perform a behavior often and still not have a habit; you can perform it rarely (a holiday-only ritual) and have one.
The shift toward automaticity is gradual. Lally et al. modeled it as a learning curve that asymptotes after a median of about 66 days for simple behaviors. Once the asymptote is reached, the behavior runs largely independently of motivation or current goals — which is the whole point and the whole problem.
In practice
Building a wanted habit means building automaticity through repetition in stable contexts. Breaking an unwanted one means disrupting the cue-routine link before automaticity does the work for you. Either way, the leverage is on the cue side, not the willpower side.
Source
See the Psychology of Habit summary (Wood & Rünger, 2016) and the habit formation summary (Lally et al., 2010).