We demonstrate that quantum mechanics entails a fundamental lower bound on the thermalization time τ of any system. At finite temperature, we show that τ is bounded by half the Planckian dissipation time, τ≥τPl/2 with τPl=ℏ/(kBT). In the low-temperature regime, our bound takes the form τ≥ℏ/Δ with Δ the spectral gap, in close connection with the quantum adiabatic theorem. These bounds, rooted in Hamiltonian estimation, hold for arbitrary quantum processes that output states close to the corresponding thermal ensemble for a nontrivial class of Hamiltonians.