Good point, you should listen to grandma
Think of coverage per square foot or watts per square foot, not number of plants. Find the recommended or best practice for your light technology and cover the area.
On your question about readiness. There are several factors already mentioned in here, they are all valid. One final factor that I use to know when the plant is ready (besides state of trichs, maturity of hairs, overall plant maturity etc.) is when the plant slows down uptake of water. e.g. if you're typically watering every 3 days when the pots are dry, at the end, if your pot/soil is still wet when you go to water and all other factors point to maturity, it's time to pull it.