Well, for pollenation to take place you need the male flowers, so it has to be toward the end of vegging and beginning flowering, when you start to see signs of sex.
And this is from the Cannabis Grow Bible:
"Female plants can be pollinated at any time but are best pollinated between 15 and 30 days into their flowering period. Plants that are pollinated less than three weeks before harvest may result in immature seeds although plants pollinated two weeks before harvest have been known to produce seeds mature enough for germination."