Depends on the pigments.
And on the pigment concentration and the actual intensity of light and the wavelengths to which the pigment is exposed. Some pigments are by nature more light sensitive, or fugitive, than others. The more dilute the pigment is relative to the body of the paint (not talking about application) has a minor effect. More important is the intensity of the light source. Most fading is attributed to the UV range, but some pigments fade more in the IR range (which is why some dyed papers fade in a warm room irregardless of light intensity). Generally, a painting should be stand in good stead under normal ambient conditions.
"Never ascribe to malice what adequately can be explained as stupidity"-- Robert J. Hanlon
“What a genius, that Picasso. It is a pity he doesn't paint.” - Marc Chagall