I think the question of how many rest days is right for you is dependent on how intense each of your workouts is, and the duration of each of the workout bouts. Also, you want to pay attention to how YOU are feeling over time.
If you are going hammer and tongs every workout session, AND if all of your sessions are all high intensity and longer than, say, 75 minutes, then a 5 workout per week schedule might be something to incorporate. You may want to consider mixing things up: one week do a 6-session schedule, the next week do a 5-session schedule, the next week a 4-session ultra-intense schedule, and then back to the beginning. Doing a change-up like that might bring in a good "body-shock" with some pleasant gains in strength and cardiovascular health.
Also, ask yourself: how is my sleep quality? It is good? Do I look forward to all of my workout sessions, and can I do them at peak performance? Is my appetite healthy, and commensurate with the amount of work that I am doing? If your sleep quality is getting poor, if you are starting to dread your workouts and feel like you're on a treadmill that you can't get off, if your performance is starting to suffer, and if your appetite is starting to deaden, then it might be time to step back and add in that second rest day.
Also, ask yourself if you are getting a good balance of cardio vs. strength. Are you mixing up intensity levels (having at least one interval session workout per week is a very good idea). And - consider starting to lift heavier on an every-other-week basis. You'll be pleasantly surprised at the benefits of that.
JMHO -
A-Jock