Montelukast typically starts to help with symptoms within 1 day to a few days after starting, but the full effect may take about a week or more for some people, especially for chronic asthma or allergy symptoms. If you’re using it to prevent exercise-induced asthma, taking it 2 hours before activity is often recommended. Always follow your doctor’s instructions and give it several weeks to assess effectiveness; if there’s no improvement after 4–8 weeks, consult your clinician. [NHS guidance and patient information commonly note these timelines]