Well, wheter it is enough or not is a matter of personal appreciation: How fast do you want to get to your results?
30 mins is normally the minimum one should to cardio, most people will say that 45 min is ideal and 60 min is the max in one session (beyond 60 mins you can count on diminishing returns and adaptation syndrome).
Since you are not lifting, careful with the catabolyc effect of cardio. Maybe a PWO shake can help (just be sure to keep that dextrose in check, important for you in this scenario is the protein not the glycogen).
I personally, do 45 min, 6 times per week @ 75 ~ 85% MHR.
If you hit a plateau, there are many things you can do:
1. Check your diet
2. Increase intensity
3. Increase duration
4. Increase frequency
Remember, there are only guidelines, I have seen people getting ripped on cardio 2 days per week (pure ectos) while others will need cardio 6 days per week... it is all very personal.