  It has long been believed that dissipative time scales $\tau$ obey a
"Planckian" bound $\tau \gtrsim \frac{\hbar}{k_{\mathrm{B}}T}$ in strongly
coupled quantum systems. Despite much circumstantial evidence, however, there
is no known $\tau$ for which this bound is universal. Here we define operator
size at finite temperature, and conjecture such a $\tau$: the time scale over
which small operators become large. All known many-body theories are consistent
with this conjecture. This proposed bound explains why previously conjectured
Planckian bounds do not always apply to weakly coupled theories, and how
Planckian time scales can be relevant to both transport and chaos.
