We present an investigation of polar cap plasma structure lifetimes. We analyze both simulated data from ionospheric models (International Reference Ionosphere model and Mass Spectrometer Incoherent Scatter model) and in-situ data from the Swarm satellite mission (the 16 Hz Advanced Plasma Density data set). We find that the theoretical prediction that E-region conductance is a predictor of F-region polar cap plasma structure lifetimes is indeed supported by both in-situ-based observations and by ionospheric models. In-situ plasma structure lifetimes correlate well with the ratio of F- to E-region conductance. We present explicit predictions of small scale (~1 km) structure lifetimes, which range from less than 1 hour during local summer to around 3 hours during local winter. We highlight a large discrepancy between the observational and theoretical scale-dependency of decay due to diffusion.