  We benchmark the decoherence of superconducting qubits to examine the
temporal stability of energy-relaxation and dephasing. By collecting statistics
during measurements spanning multiple days, we find the mean parameters
$\overline{T_{1}}$ = 49 $\mu$s and $\overline{T_{2}^{*}}$ = 95 $\mu$s, however,
both of these quantities fluctuate explaining the need for frequent
re-calibration in qubit setups. Our main finding is that fluctuations in qubit
relaxation are local to the qubit and are caused by instabilities of
near-resonant two-level-systems (TLS). Through statistical analysis, we
determine switching rates of these TLS and observe the coherent coupling
between an individual TLS and a transmon qubit. Finally, we find evidence that
the qubit's frequency stability is limited by capacitance noise. Importantly,
this produces a 0.8 ms limit on the pure dephasing which we also observe.
Collectively, these findings raise the need for performing qubit metrology to
examine the reproducibility of qubit parameters, where these fluctuations could
affect qubit gate fidelity.
