The brighter your males look, the sooner you should see result but as said above it will be atleast a month. For me I've seen black appear as soon as 2 weeks on some of my fry while others wont be coloured till 20 days later. Also lighter colours like yellow and orange on fry will take longer to really stand out.
Cheers for the replies. I read toady, that the way to sex all livebearers, is to wait until the anal fin is large enough to distinguish between being triangular in females, or more like a spike in males.