While it varies from brand to brand, the effects of most laxatives are generally eliminated within 24 hours. Most begin to lose their effectiveness within eight hours, state various HealthTap contributors.
While mineral oil and other lubricant laxatives allow for quick excretion, fiber-based laxatives and stool softeners put less stress on the body and may take up to one week to be fully effective, according to WebMD. Milk of Magnesia and other osmotic laxatives draw water from the intestine and require several glasses of liquid to reach their potential. Ex-Lax and other stimulant laxatives may result in fast discharge, though prolonged use may lead to dependency.