why are cycle dates to take Clomid different?

Destynee • PCOS • 2 natural born • pregnant with our letrozole baby🤍
Just curious why some women are told for example to start their Clomid on CD 3-7 and others 2-6 or 5-9. What is the difference? Is it for women with longer and shorter cycles? My cycles are irregular and draw out to around 40 days so I just don't know when to start.