While substructures of offshore wind turbines become older and begin to reach their design lifetimes, the relevance of measurement based lifetime extension increases. To make well-founded decisions on possible lifetime extensions, damage extrapolations based on measurements are needed. However, although for all substructures, fatigue damage calculations were conducted during the design process, there is no consensus on how to extrapolate 10-minute damages to lifetime damages. Furthermore, extrapolating damages is an uncertain process and its actual reliability is unknown. Therefore, the current work uses data of offshore strain measurements to assess different approaches of extrapolating damages, and to investigate the reliability of damage extrapolations. For the present data, the most reliable lifetime estimations are possible, if the damage data is split up into wind speed bins. For each wind speed bin, the occurrence probability should be based on data rather than on design documents. Moreover, using mean damages in each bin is the best practice. Furthermore, our results suggest that strain measurements of about 9 to 10 months lead to a relatively representative and unbiased data set. Therefore, if there are no significant changes of the turbine or the environmental conditions over the lifetime, damage extrapolations based on such a time period are sufficiently accurate.