Welcome to Just Answer. If you are having shortness of breath, you should call for help right away. If you have a pulse oximeter at home you can call your doctor with the results to see if your oxygen needs increasing.
If it is reading 70 you need to call your doctor or an ambulance right now. No one can predict when you are ready to die, or how low it can go for any one particular individual before they pass out. If your pulse ox is reading 70, then either it is malfunctioning, or you need help now. Oxygen delivery is usually titrated to give an SpO2 of 94% for most people, and of 92% for CO2 retainers. Respiratory failure usually occurs between 85-90%. Below 85% usually requires intubation and artificial respiration.