i was asking the same question for i am a newbie for flasking as well.
i was told replating can be done 2 weeks post germination, some said when the first roots appear, some internet says as long as the protocorns is at a workable size of 2-3 mm, it is ready to replate. so many different opinions but all with a common phrase, it all depends on the condition of the protocorn. if the condition of the mother flask permits, let them grow bigger before replate to increase the chance of survival.
my first mother flask was over crowded and thus they were turning into a layer of green carpet, i had no choice but to replate them when they are about 1-2mm big. no roots or leaves only protocorns but those flasks that do not suffer from contamination, so far i can see small "horns" (leaves) growing.
i am just a beginner so take my words with a LARGE pinch of salt. =D