I never veg for less than a month and most start to show by then it seems. The males always show first it seems too, so anything that isn't showing by then is usually female too.
Just my experience.
I do not trust myself to sex a young plant in vegetation. Those pre-flowers have fooled me more than once!
I have not had a non-auto plant flower in 24/0 or 18/6. ( not including pre-flowers.)
With non-female (reg seed) when grown to desired height to begin flowering there will be male and females. What a waste (in power, nutes, space and time) growing unwanted males for so long. (And they are often the nicest biggest plants.)
Instead I choose to waste only my time by:
Veging until around five nodes (about a foot tall.)
Flowering, with a very watchful eye, until sex shows. (Getting rid of unwanted males as they show their sex.)
Reveging until desired height.
Re-flowering until harvesting.
(Caution: this may on a rare (for me very rare) occasion cause hermaphrodites.