I use Mina (I'm a trainer).
Brow hairs have a lifecycle of 6-8 weeks. Mina permanently tints the brow hair but the effect fades as tinted brow hairs are shed and replaced with new untinted hair. My clients return at 4-6 week intervals depending on regrowth.
Brow skin staining isn't legal in the EU only tattooing/microblading. Brow henna is marketed as a brow hair tint - any skin staining is incidental, not an integral part of the treatment - that would be illegal and also false advertising as it isn't possible with any legal product. Your clients will be very disappointed if you market a henna brow as a skin stain - I tell mine that the staining lasts like a spray tan, and if they hate it they can scrub it off at home. I train a lot of microbladers who find that henna works very well for clients thinking about semi permanent brows and for keeping in touch between top ups.
Hena bonds to the keratin in skin, hair and nails as it oxidises, Areas of soft, smooth skin have less keratin so don't take a henna tint very well. Areas like the sides of the hands and feet have more keratin so henna will bond quite well. Some keratinised areas are subject to heavy wear (like the palms of your hands) so a henna "tattoo" will wear off quicker. This is lucky if you stain your fingers when applying henna!
Skin under the brows doesn't have much keratin. On some clients you will get very little skin bonding. The condition of the skin and the skin home care regime will all have a bearing on how long a skin tint lasts - not all the factors are under the clients control. Or yours. Some clients will get a great skin tint but the effect will wear off very quickly whilst other clients both take a skin tint and keep it for up to a week.
What you should be looking for in a brow henna is how long it takes to do a treatment, how good the colour range is and how much PPD is in the products.
PPD is in most tints, it's in Hive brow tint for instance and it's in most henna brow tints in higher concentration. PPD is a stabiliser and is a known sensitiser. It's the reason we patch test! It's essential in a long lasting dye - any substitute is so similar chemically that it makes no difference to sensitisation. The skin under the brows is more reactive than the skin on the scalp. A client might not be aware that they are slightly sensitive to hair dye - this type of client can react badly to brow henna, so a good patch test is crucial and training in how to do a good patch test is essential. I see on forums therapists asking, "why has my client reacted, I did a patch test" and it's evident that they don't understand enough about patch testing.
I wouldn't want to use a brow henna without training. Ive had a look at the "training" offered by some brands who don't require you to attend their own face to face course and frankly I wouldn't know how to use their product to get professional results, I'd have to wing it on my clients, which I'm not going to do. For this reason I've not tried any other brands. Ive looked at Facebook and seen the results - but I'm also reading the questions people ask. I want a brand that can support me.
Mina brow henna has 1% PPD in 7 colours and 2% PPD in black. I don't think I've ever used 100% black on a client, I usually mix a custom blend of 2-3 colours. A brow treatment including reshaping the brow takes 25 minutes - occasionally 30 minutes. My trainees usually book 40-45 minutes for a new client and 30 mins for a returning client until they gain confidence.
About half of my client base has now switched over to henna. I charge £32 for a henna brow and clients return monthly. I used to find it difficult to fit in all my treatments especially at peak times (lunch time and straight after work) Switching clients over to henna created more bookable hours for my clients and increased my hourly rate so everyone is happy!