It depends. I have a bath most evenings, as it's partly for relaxation and washing the day away before I sleep, and yes, I know it's not very eco friendly. Yesterday was unusual, but I got distracted by MN, and it was rather late, so I just went to bed and had a shower in the morning before work.
I shower after swimming, but I'm not sure that counts, as I've been in water anyway. Usually after yoga, unless it's the weekend and I'm home alone. I would shower if I've got particularly grimy while gardening, and I've also showered in the summer to cool down, and also when I've been having an allergic reaction to cats or pollen - then I might shower to ease the itching and wash the allergen off. I will also shower before a big night out.
Generally bath or shower once a day, though. I have been known to go two days when it's the weekend, and I'm just staying at home, and just have a strip wash instead. It's not like that takes any less time than a shower, so it's a strange way of being lazy.
I do remember when I was doing one of my DofE expeditions back in the last millenium - we were doing 3 or 4 days across Dartmoor, and on the second night, I washed my hair by the river we were camping near. I felt about a hundred times better, even though the water was rather icy. No showers though, just a good wash of the sweatier parts. We were all in the same state, so I don't think we offended each other's noses too badly.