There's different theories out there.
I personally get that endorphin rush I crave from cardio so I do a little 5 days a week. I have heavy cardio days 3 days a week (Spin class and elliptical) and light cardio days 2-3 days a week (usually running 3-4 miles).
My personal trainer recommended that I was doing too much cardio.
Maybe. But I like it...and I get my strength training in there too.