How long do the effects of citalopram (celexa) last after you stop taking it?

2+ weeks, roughly. Antidepressants take some time to start working, and they take a similar amount of time to stop working, too. This varies from about a week to a month or more. But for most people it's about 2 or 3 weeks. Note that this applies to the antidepressant effect. Side effects such as agitation or upset stomach usually start right away, and end right away too, often in a day or two after stopping.