Hi again joko. For me it's just a couple of days but then I'm only on 10mg. I occasionally don't get round to getting my repeat prescription on time too and that's how I know.
I do have to say though, especially for other people that may come along and read this, that I am on a low maintenance dose that keeps me just on the right side of slipping back into a depressive state. In fact, quite often I can get away with only taking 10mg every other day. So really I'm not the best subject for comparison.
For anyone on a higher dose, as jen78 says, it would be quite different. A week without them for someone like me on 10mg and who doesn't suffer quite as desperately as some is not too serious, but for people on higher doses (who's symptoms are obviously more severe) the effects of suddenly stopping can be quite bad and it's definitely NOT recommended.
The head 'pings' that people speak of are quite common when people suddenly stop taking Citalopram. You can also be prone to a major 'crash' in the first couple of weeks too. It is ALWAYS wise to stop them gradually.