(Disclaimer: I don't actually have experience with herbal colour/conditioning, I just find the science of it fascinating.)
Henna conditioning is twofold: the lawsone molecule (which is what gives the colour) binds to the keratin in the hair, conditioning and strengthening it structurally; this is usually permanent. The other part is the waxy/fatty acids from the outside of the leaf which coat the hair and provide some conditioning/strengthening benefits; this is temporary and wears off within a few weeks (depending on how and how often you wash your hair).
The conditioning from cassia is the same as the waxy/fatty acids from the henna; chrysophanic acid is the compound that gives the colour and that wears off within a few weeks (again, depending on your routine).
[Incidentally, I know you didn't mention it but sedr works the same as cassia but without the colour, as far as I understand. It's not often mentioned because people often want colour as well as conditioning.]
Our resident expert is
Nightshade who can explain further if need be and fill in anything I've missed
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