Depends. Ventricular tachycardia in the presence of structural hear disease is at risk of sudden death. However V Tach in a normal heart or without underlying heart disease is benign. It can cause palpitation but the risk of death is low. It often originates in the right side of the heart and can be treated with medicine or cured by ablation.
Highly variable. Vt can be non-sustained which isn't generally life threatening or sustained. Sustained VT can occur at different heart rates. Relatively slow VT (what rate will actually be tolerated is dependent on many factors) can go indefinitely. Vt associated with low blood pressure and shock will result in death in seconds to minutes. Any VT can deteriorate so it's always considered "potentially lethal".