If you want a really nice smoke then leave her growing untill at least Half of those White hairs have started turning Brown/Red.
You will also get more bang for your buck as she'll grow a bit bigger and produce a bit more bud
If you look at the photo of the bubblicious auto on the nirvana website you'll see the colour of a finished girl. Thats what you should be aiming for
Hope it all goes well for you