All cannabis is sativa. Just a heads up.
You want the heads of your trichomes to be cloudy. If they are clear, they are immature and won't do much of anything. If they are amber, they are overripe.
Amber heads mean you'll get a couch lock kind of effect if you have 20%+ amber trichomes and the rest are cloudy.
If you have 95%-100% cloudy, you'll get the full effect of the mixture of cannabinoids. This is ideally what you want to shoot for, but it rarely works out this way.
Personally, I like waiting until there's 15% amber heads and the rest are cloudy. It gives me a nice, enjoyable, relaxing high.
But, it all comes down to your preference.