When do you get your roots dyed?

Since I have dyed hair, I know I need to get my roots touched up periodically, but because I don't have time to go to the salon, I keep putting it off, and before I know it, a lot of black hair has grown in, haha. However, it seems like the extent of root growth varies from person to person; to me, it looks like I need to go get a touch-up right away, but then some people around me say they don't need to go that early and tell me to wait a little longer, haha.

I'm curious how long you usually wait between root touch-ups!