I used 100g of pure henna and left on for just under 2 hours.
The final effect...
This is about 4 hours after I finished rinsing the henna out. It will settle down over the next few days and become deeper and richer.
I still haven't had it cut since September and it is starting to look slightly tatty on the very ends. But definitely not as bad as you would expect for 8 months growth.