Different bases do different things with reserve. Some bases, reserve is a full week thing (like flyboyknight said, usually covered by the senior pilots).

At my base, we split up reserve (typically, if there are enough pilots you can plan on at least one reserve day a week). One run typically blows out, so the junior reserve pilot usually does an out and back to cover the excess (if they don't cover a sick call first).
The senior reserve pilot has a much better chance of going home early without flying.