ive seen people say anywhere from a few days, to a week or two
eek
but it probably has a lot to do with how well you stretch the strings.
you can actually get string settling confined to just a day or two. if you have ever strung a classical string guitar then you probably already have an idea of what you should do, nukedoc shows a similar technique for string stretching in a video where you simply hook your finger under a string, tune it up about a half step, or a whole, but not much more out of risk of strings breaking, and pull up on the string slowly for a few inches, or until its pretty tight on your finger, rinse repeat
these guys are doing some pretty tame stretching, nylon can be pulled up a bit higher
that ought to take out a lot of the slack in the strings, just take it slow so you dont snap a string in the process. also remember to always tune UP to youre note, as opposed to down. with a steel string acoustic its a lot less of an issue, but with nylon it basically ensures you will get a flat string after a few minutes of playing if you tune down to pitch as opposed to up to pitch. if you miss your mark, always tune down a little and retune up
anyways, you can always expect slippage on new strings, but itll go away with more playing time!