Totally depends on pretty much everything, could be as soon as a couple months, could be years, depends on the species, conditions, you know, all that jazz... I have some D.tokaiensis (my first CPs from seed) that look like fully mature plants (albeit small-ish) which are maybe 6 months old, some others I planted a few weeks later of different species are still looking sorta microscopic in the same conditions
I know this isn't what you want to hear, but it is the sad truth
Do you know the species you're working with??