If I may contribute something to this thread..
Generally, if harvesting greens, it is best to take leaf (no more than one third of the plant at once) up until the point when flowers begin to manifest. At that point, the plant is beginning to divert energy and nutrition away from the leaf and into the flowering parts, with the end goal of producing seed. If you desire flowers or seed, that is a good time to stop harvesting leaf. The height of the plant at that stage will of course differ widely between genera and species (and even within species varieties) and depending on sun, soil type, climate, nutrients, water, etc. So the rule of thumb whereby you harvest up until flowering will allow you to get your greens while still allowing the plant to manifest its physiological distinctiveness and reproduce.
Peace and love,
- "All that is gold does not glitter,
- Not all those who wander are lost;
- The old that is strong does not wither,
- Deep roots are not reached by the frost.
- From the ashes a fire shall be woken,
- A light from the shadows shall spring;
- Renewed shall be blade that was broken,
The crownless again shall be king."
~ J.R.R. Tolkien