If I don’t rinse it, my hair will end up black and I’ll look like Dot Cotton

This evening, she surprises me at 8pm when I am in the bathroom. “I’m dying my hair,” I say, “I’ll call you back.” “Oh,” she says, “any exciting colour?” “No,” I say, “just the usual dark brown for the roots. But I lost track of time, it’s already been on for longer than 15 minutes and if I don’t rinse it out now, my hair will end up black and I’ll look like Dot Cotton.”
“Blasted nuisance having very dark hair,” she says, “if you keep dying it to get rid of the grey, once you get to a certain age, you just move into Dot Cotton territory with every year that passes.” “Or in this case, every second,” I say, checking the mirror for signs of Dot, “the timing is crucial, mum. I’m going to have to go.”