1. We can take showers.

2. We can take a shower.

3. We can take cold showers.
4. We can take a cold shower.

5. We can take longer showers.
6. We can take a long shower.
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What is the difference between each pair? Which one is commonly used? Does each pair have diference in meaning?

I'd use 1, 3, and 5 -- mainly because "we" is plural, and it would be odd for several people to take "a shower."

=)