Here’s an excerpt from the science-y hair blog about when to add protein:

Whose hair needs the extra boost protein can offer? As a generalization: ©Science-y Hair Blog 2013

People whose hair is losing more water than it should - porous or damaged hair.

That means:

Bleached or highlighted hair

Chemically relaxed or permanent waved hair

Hair that has been frequently brushed

Hair you take swimming frequently in chlorinated pools or salt water

Hair that spends several hours each week under the full sun (winter in higher latitudes might not matter as much)

Some hair that tends to be dry no matter how much oil and conditioner you use

Hair that has been styled with high heat - flat irons, curling irons, hair dryers on “hot.”

People whose hair needs conditioning, but cannot take a lot of added weight. That’s you, fine-to-medium-haired people, or anybody with silky, soft hair. Protein can bulk your hair up a little and add strength.

Who needs to approach protein with caution?

If your hair is quite coarse (wide hairs), it may not tolerate protein very often. Or it may tolerate the smaller proteins and amino acids.

Coarse but porous hair may tolerate (and benefit from) protein more than coarse, lower porosity hair.

Referenced from:

https://science-yhairblog.blogspot.com/2013/09/more-about-protein.html