We obtain new estimates for the existence time of the maximal solutions to the nonlinear heat equation ∂tu − ∆u = |u| α u, α > 0 with initial values in Lebesgue, weighted Lebesgue spaces or measures. Non-regular, sign-changing, as well as non polynomial decaying initial data are considered. The proofs of the lower-bound estimates of lifespan are based on the local construction of solutions. The proofs of the upper-bounds exploit a well-known necessary condition for the existence of nonnegative solutions. In addition, we establish new results for lifespan using dilation methods and we give new lifespan estimates for Hardy-Hénon parabolic equations.