How do you know...

[SlightlySoprano]

...when your hair needs a protein treatment? Is it easy to mix up protein deprived hair and just plain old dry hair?

[ghost]

I used to give myself protein treatments fairly often, but I was bleaching my hair at the time.
From what I've gathered, if you have healthy, pretty much undamaged hair, a protein treatment isn't a bad idea for a once a month kind of thing. 2-3 times is good if you have damaged hair.
Be sure to get some moisture into your hair afterward; they can be a little drying at first but will make your hair stronger. I liked Aphogee products for mine.