We let our house to a friend for about 18 months. When they left it was grim. The bathroom grouting had gone orange (mosaic tiles so a right pita) and I used Harpic and scrubbed the toilet several times to get it clean.
It's made me a bit judgey about bathroom hygiene tbh. I work full time and don't get round to cleaning every week, but it never gets like that. I notice it quite often at other people's houses. How long does it take for that orange bacteria to form? And for a toilet to go dark green?