that would depend on the laxative type and dosage. most laxatives act relatively quickly and then dissipate. some pledge to "work by 8am the next morning" if taken at night. i guess that would be good as long as you don't oversleep! the only longer lasting laxatives would be the bulk types like those containing methylcellulose and other undigestible fiber. some laxatives are quite potent and will literally purge the colon. these are called cathartics. these are best to avoid unless you are preparing for a colonoscopy.
how long does a laxative last