It's really going to depend on a number of variables: i.e. the strain of seed, how good the growing medium is, ph of the soil, temperature conditions, lighting etc.

My current grow showed sex at 17 days (2.5 weeks!), so it's not always easy to predict, however if you take a look at some of the pictures posted on the site you'll see it's relatively easy to tell which is which (well before you need to worry about cross pollination anyhow).