As a general rule, furniture with oil and wax finishes or dyes and wax should be oiled once or twice a year, or whenever the wood looks particularly dry. To maintain the silky look and feel of oiled wood, regular re-oiling is recommended. It should be re-oiled when the wood starts to look dry or discolored. Re-oiling once a year is usually sufficient.
Most modern finishes, and by this I mean since the mid-18th century, are some type of barrier layer that is applied to wood to protect it from moisture and abrasion and, secondly, to enhance the beauty of the wood.