How often do you do just roots vs full head?

I’ve been dying my hair with conventional black dye for like ~7 years and I now have 2 full head henndigo applications under my belt. I love henna for its hair health benefits but my hair is super long so doing a full head henndigo is a lot of work. I’m wondering how often you guys do just roots vs full head? Do you alternate every other time, every two times, or do you just do roots? TIA!