That's an interesting question.So the standard's one shower a day in the Western urban world, or so it seems. But another question is how long it takes a shower in different parts of the world. I am amazed that some people can have a shower in less than 5 minutes, including all the dressing and undressing and so on. I never stay less than 30 minutes under the shower myself.
I grew up (and still live) in an area without municipal water, which means each home has its own well (which is connected to the house's plumbing system, of course). Our old well would often run out of water, most often in the Spring, but the risk was always there. Therefore, I grew up with 5 minute showers being the norm. We have a new well and I can stay in longer, but I doubt it is normally more than 8 minutes. The times I am citing are for actually running the water and bathing alone, not drying off, shaving, getting dressed, etc.