you can get a good idea at a couple weeks, but on rose combed birds it's a lot harder to be accurate than say a single combed chick.
My phoenix and ohiki, you can tell in just a few days, that single comb will start to poke out on the head pretty quick.
Depending on the growth rate of the individual in rose combed birds, they will trick you a lot. Some pullets get good combs, some virtually none. So the good combed girls can at time look like cockerels based on comb, BUT it's usually paler.
At about 4 weeks, if you see some with decent bigger than others combs, and it's a darker pink or red. it a good shot that it's a boy.
At this age if they dont have much of anything, good shot it's a girl.
But as many as I have raised, I still dont count them til they get a little bigger, feather type or color patterns on certian varieties are the best most reliable way to sex them. It just takes longer to get to that point