Henna is permanent permanent permanent! Unless you have very odd hair (and some do), it is *extremely* unlikely that henna will *ever* wash out!
Some people have done henna glosses which have sloooowly faded, but others find even these to be extremely permanent.
I find that the only thing henna has ever stained is my skin and my hair. It washed right off the porcelain bowl and my bath (not procelain...not sure what it is). The only thing I would expect henna to stain is cheap, porous plastic.
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