Depends on what type of high you like. Harvesting when your Trichs are all milky, before Amber, gives more of an energetic high. If you wait until your Trichs are mostly Amber it gives the couch lock high. No microscope, then you should harvest when 2/3 of your hairs are red. Every strain is different though, so this general rule of thumb may not be right for everyone, and a pocket microscope is only $10-$20 and gives you the most accurate reading on when your plants are ready.