The thermal lifetime assessment of electrical machines' insulation systems is generally performed at the design and prototyping stages. Hence, the insulation system's thermal endurance curve (or temperature index) can be extrapolated, allowing the machine designer to tune lifetime prediction models and / or qualify a market-ready prototype, according to technical standards. The whole thermal evaluation process usually relies on accelerated thermal aging tests carried out on appropriate specimens, and an extensive economical effort could be needed. In this paper, a methodology relying on the Weibull statistical distribution and the geometrical size factor (i.e. statistical enlargement law) is analyzed for evaluating the machine's thermal life by using simple twisted pair as specimen. The obtained results are also experimentally validated against data collected on a different specimen's layout, namely random wound coils, confirming the feasibility of the analyzed approach.