Twisted string actuators are widely used in different areas of robotics, however, their lifetime remains one of the main concerns for robotics. There are no comprehensive studies that relate the lifetime of a twisted string to its material properties and operation conditions, which can be important to improve the life cycle. This paper provides preliminary results of an extensive experimental study on twisted string actuators, using strings made of different materials and operating in various working conditions. We experimentally investigated the life time of two different strings and provided empiric mathematical model which relates operating conditions of TSAs and string material and properties to their endurance. Based on experimental results, the observations are made that the life cycle of the strings decreases exponentially with the increase in load forces and stroke and increases linearly with the increase in the number of strings.