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In this paper we study several semilinear damped wave equations with "subcritical" nonlinearities, focusing on demonstrating lifespan estimates for energy solutions. Our main concern is on equations with scale-invariant damping and mass. Under different assumptions imposed on the initial data, lifespan estimates from above are clearly showed. The key fact is that we find "transition surfaces", which distinguish lifespan estimates between "wave-like" and "heat-like" behaviours. Moreover we conjecture that the lifespan estimates on the "transition surfaces" can be logarithmically improved. As direct consequences, we reorganize the blow-up results and lifespan estimates for the massless case in which the "transition surfaces" degenerate to "transition curves". Furthermore, we obtain improved lifespan estimates in one space dimension, comparing to the known results. We also study semilinear wave equations with the scattering damping and negative mass term, and find that if the decay rate of the mass term equals to 2, the lifespan estimate is the same as one special case of the equations with the scale-invariant damping and positive mass. The main strategy of the proof consists of a Kato's type lemma in integral form, which is established by iteration argument.