They can be a good bit shorter than caliber length. I have some loads that only use a third of the neck. I test them by pushing the bullet against a piece of scrap wood pretty hard and if they don't move I'll leave one in the magazine for 10-20 shots then again check for any movement.
If they do get shorter I will try more neck tension and repeat the exercise.
The one caliber or more neck length is a good standard for case design but not a hard and fast rule for seating bullets.
The main requirements for bullet seating are that it is straight and it doesn't shift.
Hence the term, "good practice". How's the runout using 1/3 of the neck?
