In many cases, plastic components are subjected to long-term loads. Thermoplastics generally exhibit viscous behavior. This means that the deformation behavior is time-dependent with respect to both elastic (viscoelastic) and plastic (viscoplastic) properties. Furthermore, the failure behavior is also time-dependent. This means that in order to perform a valid FEM simulation, material data on the long-term deformation and failure behavior of the material are required. However, these input data are usually not available or only to a very limited extent. The reason for this is the very large amount of effort required for their experimental determination. Such data are usually determined by means of long-term creep tests, which are naturally very time-consuming and cost-intensive: For the deformation behavior, numerous tests with a duration of 1,000 to 10,000 hours each (i.e. 1.5 months to slightly more than a year) are usually necessary. In this paper, a method is presented that enables long-term material data to be determined in a time- and cost-efficient manner. Isochronous stress-strain diagrams and short-time tensile tests serve as a basis for this. These data are often available from publicly accessible sources. Phenomenological spring-damper models (Maxwell, Kelvin-Voigt) are used in the investigations to represent the time-dependent deformation behavior. Energy or creep rate criteria are used to describe the failure behavior. The work is exemplified for a polypropylene and different levels of creep stresses. In addition to the secondary creep phase, the tertiary creep phase up to the onset of failure is also described. This allows the determination of a creep curve, which is decisive for the evaluation of the obtained simulation results with respect to a failure onset. Within the scope of the paper, the computational results obtained with the material models are presented and their validity is evaluated by comparison with corresponding measured data. The long-term material data are determined experimentally with reduced effort using the so-called Stepped Isostress Method (SSM). For each creep stress, this time-rapid stepped method requires only one short test (order of magnitude 24 to 48 hours) on a representative specimen with stepwise increase of the creep stress to form a master curve for times greater than 10,000 hours by means of the so-called Time-Stress-Superposition Principle (TSSP). These accelerated tests can be carried out until failure occurs, so that the resulting master curve also describes the time-dependent failure behavior. Currently available first results show that a computational estimation of the deformation and failure behavior based on the mentioned models seems possible. Furthermore, the applied experimental procedure also shows that it is suitable for determining long-term material data with comparatively little effort.