If a variegated plant starts to grow mostly white, how long can it survive? How many green leaves/how much chlorophyll does a plant need in order to keep growing?

Especially the discussion of selenium. One thing, “variegated plants” covers a lot of territory. It’s often difficult to make generalizations about “plants” - there are so many, and so many differences among them.

Variegation, in most cases, is an adaptation that developed to help plants survive in an environment where the amount of light has increased from the levels of the originating environment - the white or yellow spaces represent areas with reduced chlorophyll, which means the plant is lowering its metabolism to some degree. When the environmental light reduces, often the variegation in new growth becomes more muted, even disappears entirely.

Leaves, or stems, of pure white aren’t unusual in many varieties of variegated plants - the nature of variegation allows the plant to “try” different pigment combinations, so just by chance, occasionally total white comes up, just like total green sometimes comes up. The adaptability of plants capable of variegation allows them to “try” different combinations of responses to environment, and what works best is what gets passed on.

Obviously, if a plant starts to grow without any chlorophyll at all, it’s not going to live very long. A few leaves or even branches without chlorophyll don’t mean much - the rest of the plant “supports” the “mutants,” but if the whole plants starts to lose color, or all the new growth is lacking in chlorophyll, there are problems going on. What those problems are, or how they can be remedied, depends on type of plant and environment.

How much chlorophyll a plant needs to survive is one of those questions that has no answer, because it depends on the type of plant.