It is best to learn the signs and let your hair tell you when you need it. It doesn't always work well - your hair might throw you a curveball now and then. (See my post in this forum about rubbery hair - ugh!) But, for example, you do a stretch test when you get your hair wet, after you condition, after you dry it and use protein products as needed throughout that process. Alternatively, put your hair on a schedule but let your hair dictate the schedule - it might need a weekly treatment, a bimonthly, etc. That's the approach I took and it seemed i needed a weekly treatment. But I recently had an issue because before I knew it several weeks went by and I ended up with hair screaming for some protein. Maybe I need to heed my own advice and just test it every time I get it wet.

Last, consider carefully what treatment to use - my "PT" consists of a protein rich conditioner that I put on and work through then rinse out - I never leave it on for any length of time and I never do a PT in the true sense of the word (egg, gelatin, or commercially prepared PT) - any of those would destroy my hair.

Good luck!
remember, with all advice or suggestions for your hair, YMMV!!
My hair:
2b wavy, medium texture, low porosity, med-high density. Protein finicky.
My routine:
Cleansing: DCNP, every 4-5 days (occasionally use CJ cleansers).
RO: CJSC, occasionally use JCDC
Gel: KCCC, CJCIAB for 2nd day hair (sometimes third)

Heat is my friend: warm water rinses, steam caps, and diffuse dry. I love CLU cloths and my Denman.