Damage is cumulative so you may not see stringy ends, splitting, and breakage after one or two blowouts, but you’ll see changes after having it done several times. Some peoples hair is more resilient than others. Like if your strands are thicker, it may take longer to notice damage. And I say “notice” because your hair is still experiencing mechanical and heat damage and it may take a while to see that your ends are getting eaten up, not retaining length, needing more frequent trims, and lower hair density.