Do you know where the seeds came from? Does the breeder have harvest date recommendations?
What strain is this?
Inspecting the trichomes for color is the single best source for determining when it is right to harvest. With a zoom feature or jewelers loupe, inspect the trichome heads (little balls on stalks). General rule of thumb is to harvest when they are 10% amber, 80% milky and 10% clear. Once you gain experience, you will develop your own preference.