for production the clon is more easy way for good, secure, high and fast yield,
15-20 days for cloning and make the root,
later
depending to strain 7-10days to veg is a minimum for root expansion in the flower recipe,
and ready to flower,
some strains are very fast from seed too and similar veg to clon, 20-25 days ready to flower,
but is more unknown the yield and quality
To be purely technical, its quite possible to flower it the instant it has roots since its already sexually mature. You can 12/12 from seed but it doesn't kick into flower instantly.