Most electrochemical measurements — and virtually all corrosion measurements — are performed only after the open circuit potential (OCP) has stabilized. Ideally, the OCP should represent the true equilibrium potential of the electrochemical system. In practice, it is sufficient for it to reach a steady state, meaning no net charge is being transferred through the external circuit (the first-kind conductor connecting the electrodes).
If measurements are taken before a steady state is reached, the electrode potential at the start and end of the experiment will differ. The measurement may still yield a result, but its interpretation will be unreliable or simply wrong. In practice, there is no universally accepted criterion for “sufficient” stability. Based on my own empirical experience, I consider the potential adequately stable when it drifts by no more than 5 mV over 10–15 minutes — a window that typically corresponds to the duration of a single measurement. This guideline applies to aqueous systems at room temperature. For high-temperature molten salt systems (700–800 °C), I have worked with stability criteria as loose as 50 mV over 10 minutes, which reflects the very different kinetics involved.
The time required to reach a stable OCP varies enormously and is highly system-dependent. In my experience, it is almost never less than 10–15 minutes. The longest I have personally encountered was two weeks — though in most practical cases you are looking at tens of minutes to a few hours. The bottom line: do not begin a measurement if the OCP has not stabilized. Furthermore, in many experimental protocols it makes sense to allow a 10–15 minute rest period between measurements, giving the system time to return to a stable open-circuit condition before the next perturbation.