If you’ve been doing 20 minutes of steady state cardio three times per week and your fat loss results have slowed down, it’s safe to say that one of the reasons you’ve reached a plateau is because you’re just not doing enough.
For most people aiming for a traffic-stopping, totally shredded physique, I recommend doing high intensity interval training (HIIT) three times per week and steady state cardio about three times per week. As for volume per session, 20 minutes total is a good rule of thumb for HIIT, while at least 30 minutes is optimal for steady state work.
You definitely need to reevaluate your progress every couple of weeks. If you’re not progressing after two weeks, add five minutes to each cardio session and fine-tune your diet even more. At some point, you may also need to bump up the frequency of your steady state sessions, and maybe even add another HIIT session per week.
Going up to as many as seven hour-long sessions per week might be necessary to achieve the freaky ripped, vascular leanness you desire. However, I wouldn’t increase the HIIT to more than four (maybe five) days per week.
The good news is that it’s harder to get lean than it is to stay lean. So, while you may have to do what seems like endless amounts of cardio to get ripped, it won’t take near that much to keep your condition.