It's a tricky question to answer because I think the place you're coming from is slightly askew. If your objective is to completely capture the shower - the movement of your body as you shower, the warmth, the feel of the water, etc. - then a longer shower would naturally be more beneficial as it will give you greater scope to explore the experience of the shower.
However, the objective of mindfulness is not to explore showers, it is to capture what
is. If speed is what you're doing, then be mindful of the speed. Showers are nice things to be mindful of, but if you've shaved 7 minutes of your shower, simply notice what it's like to have a shower in 3 minutes then be mindful of jumping out into a cold bathroom and rushing your clothes on in the next 7.
The object of mindfulness is not so important, it is the intention, action and attitude that count.
However, as I said, showers are nice things to be mindful of, so if you can have a longer one to allow yourself to be mindful of it, then by all means feel free. Try not to worry about whether what you're doing is right or not, just go with the flow and let your practice be what it is