Every 4-5 weeks. I use l'oreal castings, which is a semi and supposed to wash out after 24 washes. I've never let it go that long though. I don't use a permanent colour due to the more obvious line it leaves in the hair.
I've never let my hair grow out, but assume its at least 50% grey. The above still works great, I buy when its 2 for £12 and my hair is a chestnut colour.