It may seem like oil paints never dry, but they actually do. Oil paints are created most commonly by milling pigments with linseed or safflower oil. These are known as drying oils, natural plant oils that dry to a solid film when exposed to air.

Drying Times

Oil paints generally dry within one to two weeks. This estimate varies depending on the ingredients used in the paint and the thickness of the paint application.

Considerations

Colors known to dry more rapidly than average are the siennas and umbers, flake white and cobalt blue. Colors that dry particularly slowly are lamp and ivory black, titanium white and alizarin crimson.

Additives

Adding a drying oil to the paint can alter its drying time. Refined linseed oil and stand oil increase drying times. Poppy oil and most other types of linseed oil reduce drying times. Painting medium and alkyd-based mediums also affect drying times.

Solution

Oil paints dry through an oxidation process. Moisture in the air can slow down oxidation. Use of a fan or dehumidifier in the room can speed up oil paint drying time.

Alkyd Paints

Alkyd oil paints contain alkyd resin, which greatly reduces drying time. Most colors will be touch dry within 12 to 48 hours. Many paint manufacturers carry a line of artist-quality alkyd oil paints in a full range of colors.

Source:

Encyclopaedia Britannica: drying oil

Winsor & Newton: Watching Paint Dry

Wet Canvas: Oil Paint Drying Times

Resource:

Winsor & Newton: Drying Oils

Winsor & Newton: Mediums

True Art: Binders--Oil-Modified Alkyd Resin