Henna gurus! Shortest time you'd leave it?

[QuimReaper]

It's been on for 4hrs - I normally go for at least 6 but does it really make a difference?

I put on a henna/indigo mix last night and only managed to leave it on for a couple of hours before bedtime. Seems to have worked ok. I had mixed the henna with lemon and warm water to release the colour the night before though.. Don't tend to leave it on for more than 3 hrs generally.