Permanent Hair Colour

How long does a permanent last?

Aprrox how many washes? I understand that it differs from person to person but what is the average amount of washes or time?

I have long hair almost down to my waist, its pretty thick with lots of layers and currently naturally black.

I'll be dying it to brown (not sure what shade) in a couple of weeks and was just curious how long the permanent would last for.

Thanks a bunch