I hear taking long showers in foreign countries is looked down upon, especially if you are living with someone temporarily. Is there a standard shower time in Spain that is considered acceptable?
I am pretty sure there is no explicit convention on this and if there is it is not due to water use - water is not that expensive and showers are a relatively economic use of water (compared to baths or even just flushing the toilet).
What's more a likely factor to offend people you live with is blocking the bathroom for a long time, or if hot water comes from a limited reservoir (I've seen this in a house heated by solar collectors) then a long shower may leave the next person with only cold water. In either case the acceptable time to take depends on the number of people.
But I think that 5 minutes would be considered reasonable in nearly all cases.
Like Michael Borgwardt said, there is no explicit convention on the subject. There are some places in Spain though with real problems concerning water supply (remember that in some places in Spain it doesn't rain that much).
I would recommend that you just ask the people you are living with if there is some water supply problems. If not, just take in consideration what Michael said about blocking the bathroom (that is annoying in every country I've been to).
Anyway, it would not hurt to " turn on the shower, get wet, then turn it off, soap and shampoo up for as long as needed before turning the water back on and rinsing off"... I think that should be a basic behaviour (water conservation habits, help the environment...) I mean, that way you don't waste water while "soaping and shampooing"...