The relationship between observed variability time and emission region geometry is explored for the case of emission by relativistic jets. The approximate formula for the jet-frame size of the emission region, $R=DcDelta t_{rm obs}$ is shown to lead to large systematic errors when used together with observed luminosity and assumed or estimated Doppler factor $D$ to estimate the jet-frame photon energy density. These results have implications for AGN models in which low-energy photons are targets for interaction of high energy particles and photons, e.g. synchrotron-self Compton models and hadronic blazar models, as well as models of intra-day variable sources in which the photon energy density imposes a brightness temperature limit through Compton scattering. The actual relationship between emission region geometry and observed variability is discussed for a variety of geometries including cylinders, spheroids, bent, helical and conical jet structures, and intrinsic variability models including shock excitation. The effects of time delays due to finite particle acceleration and radiation time scales are also discussed.
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