I'm not an expert of Henna...but I did do alot of research on it b4 doing my first treatment yesterday... I used Reshma henna, and the packaging says to let it sit for 3 hrs b4 application to allow for dye release. I allowed mine to sit overnight...then applied it to my hair in the morning and left it on for 4.5 hrs. I naturally have dark brown to black hair.... And after my henna treatment the color didn't change much but it definitely has a copper-reddish glow in the sunlight...aside from color my hair came out REALLY soft and once dry my curls were gorgeous! Hope this helps
You will have to experiment and see because everyones hair is different. I used to keep my henna in my hair for around four hours but figured out that I can now get away with keeping it in for a little over an hour. It also depends on which henna you are using because some deposit more color than others.
Dye release will depend on the henna and the ambient temperature. Cooler temps = longer dye release; warmer temps = quicker dye release. Some henna batches release more quickly than others. It may take some experimentation with a batch to find the best point of dye release, but in 72F ambient temperature, a reasonable starting point is to assume 6-12 hours for peak dye release. You can generally tell because the wad of goo will deposit an orange stain on a paper towel or your finger.
If you want the deepest, reddest color a particular henna batch will yield, you want to try and catch the dye release at it's midpoint, or peak, so that it continues releasing while it's on your head.
All henna turns orange/red. It's what it does. If you don't want orange/red, you should probably stay away from henna, unless you have naturally dark hair.
Also note that dye release and application duration are 2 separate concerns. If you apply BAQ henna immediately after you mix it up, you would require several hours of application duration. Because it can't dye your hair until the dye starts to release, and your application must account for that. Most people mix up BAQ henna x hours before application, and then figure out their application duration from there.