Optimum time is usually about 80%+ red/orange hairs (pistils). However, this is not the most reliable method.
A better method is to use a pocket microscope or jeweller's loupe (the eyepiece they use to inspect gems) to check the trichomes. You will need something with at least 10x magnification, preferably more.
Check the Growfaq for more details, specifically
this page and
this page.
Edit: You can also DIY a ghetto style trich scope, using a mobile phone camera, and a lens from a CD player.
See here for details.