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8-10 wk after they're born. Once you can see their tailfin pattern you'll start noticing the males develop their ventral and dorsal fins. The señoritas start looking longer around that time.
Look for the tail fin shape. The males will have a more pointed anal fin as they develop.
Also, patience is the key! I was convinced my guppy dropped two batches of all female fry (50+ total) when she did, but after 2-3 months the gender of the fry were apparent