Generally you’ll want to add two or three handfuls of wood chips to an electric smoker every 45 minutes once the previous batch has fully burned out, for the first 4-6 hours of actual smoking that you’ll be doing. If you find they’re getting used up before that, soak a batch in water for 45 minutes in the meantime.
Pro tip: Add the wood early, while the meat is still cool – and do it fast – so the last amount of heat possible escapes.