Why there is no single number
Dogs learn at different speeds. A confident, food-motivated dog may be searching boxes in a week. A nervous dog may need a month just to approach the first box. Neither is behind. They are starting from different places.
Your goal matters too. Teaching a dog to enjoy sniffing boxes at home is a different project from training a dog to compete in a trial, and both are different from training a working detection dog for a job. Professional programs have their own schedules, and those vary by agency and by the odor the dog is learning.
So use the stages below as a map, not a deadline.
One more thing to keep in mind: age, breed and past experience shape the start. A dog who has been rewarded for sniffing all their life jumps in. A dog who was told to stop sniffing on walks may need time to believe the nose is allowed. Give them that time. Rushing a dog who is unsure only adds pressure.