As long as you challenge yourself and MAKE IT HURT.
I hate seeing people at the Y sitting on a bike and pedalling away on the lowest setting reading a magazine. These people do not get results To lose weight, you have to raise your heart rate and keep it high for at least 10 minutes (this is the definition of cardio) and cardio is what burns fat.
3-5 times a week is ok, but make sure you are getting a GOOD workout. Meaning one that makes you tired when you're done.