Introduction: Hormonal contraceptive (HC) use has been associated with reduced risk of asthma onset, but it remains unclear whether this protection accumulates with prolonged use or varies over time. Aims: To characterise the temporal pattern of protection against asthma onset associated with HC use, and to assess whether this pattern differs by route of administration. Methods: We constructed a 15-year (2005–2019) open cohort of women aged 16–45 years from the Clinical Practice Research Datalink (N = 2,110,199). HC exposure was derived from prescription records and classified annually. Cox regression models compared alternative exposure definitions, including current use, static lifetime duration, and episode-based duration defined as time since initiation with resetting after discontinuation. Episode-based analyses were conducted overall and repeated by route of administration (oral vs other). Results: Current HC use was associated with a reduced hazard of asthma onset (HR 0.74, 95% CI 0.72–0.75). Static lifetime duration suggested a strong dose–response (≥5 years: HR 0.32, 95% CI 0.31–0.33). In contrast, episode-based analyses showed that protection against asthma onset was strongest shortly after initiation. For oral HC, the hazard of asthma onset was lowest in years 1–2 (HR 0.64, 95% CI 0.62–0.65) and attenuated with duration (≥5 years: HR 0.67, 95% CI 0.64–0.71). HC via other routes showed a similar early reduction (years 1–2: HR 0.72, 95% CI 0.66–0.79) but attenuated more rapidly toward the null. Conclusions: Protection against asthma onset associated with HC use is strongest shortly after initiation and does not appear cumulative. Apparent lifetime dose–responses likely reflect aggregation of repeated early treatment periods across an individual’s prescription history.