I would never limit a guest on the utilities they used in my home (though yours sound as though they are very much taking the piss with the petrol money, OP! Are they students or something?), but DP and I live in the south west, and we are extremely careful with our water. We're down to £25 per month for two of us, but we use loads of water-saving techniques. You're going to think we're nutters, but water is a precious resource!... As is money...
We run a dehumidifer (live near the sea/loads of rain, thus constant battle with damp). All the water collected in that is funneled into empty bottles to be used for the flowers/washing the car/ironing/etc. Also, since it's ionised, it's brilliant as a basic cleaning fluid.
Same with water from the tumble dryer tank.
When we run the shower, it takes a few moments to warm up. So we stick a bucket under the flow for that time until the water is heated. Bucket is kept in the bathroom to flush the loo with! (Obviously only for the two of us, wouldn't dream of making a guest do that!)
No hoses, no dishwashers... Though I will totally be a hypocrite and build a swimming pool when I make it rich...
And absolutely NO washing dishes under a running tap.
Water butt in the garden to water the edible plants with (you shouldn't use ionised water on anything you plan to eat).
(And I am also tight enough to take full advantage of my posh gym membership to wash my hair there several times a week when I'm done working out!)
I do think any shower over 10mins is ridiculously excessive, and I have long hair right down my back (growing out of my head, obviously, I don't mean I'm some kind of freaky bear-woman). Even if you need to shave/exfoliate, then why not stop the water whilst you do that? Then rinse afterwards.
But then, I balk at people who don't fill their washing machines properly. The world will soon become so overpopulated that there won't be enough water to go round, why waste it because you're brushing your teeth with the tap running?