Ventricular tachycardia is indeed potentially lethal, and in just a few minutes. Depends on the heart's overall condition and history. SVT is not lethal, and many live with it largely untreated for years. It's the symptoms that make the difference. Some patients don't know they're fibrillating or in flutter, but the ECG will show it clearly. Others know because they feel faint, breathless, or they feel the thumping of the irregularly irregular beats of the chaotic atrium beating out of step with the ventricles below it.

SVT, which involves the upper chambers, over time can lead to heart failure but not likely "sudden death." That would be ventricular tachycardia. You may have misunderstood something the doctor said about your condition, may want to confirm with him or her, as thinking about "sudden death" would certainly be distressing.