I am not 100% on this but IMO
they are ready to go when you see it...
It is a daily battle for me to sort my swordtail juvis..I have 9 varieties and some show gonopodium in 2 months and others still nothing after 4-5..
There are also 'late bloomers' that will stay hidden males due to [IMO
] Growth Inhibiting Hormones produced by the alpha males...Possibly as long as a year !