We don't control how long the curators keep songs in their playlists, that's totally up to them.
Some will remove tracks after one week or some leave tracks in there for several months.
If a curators really likes your song a lot changes are he will leave it in there for a longer time.
But some curators refresh their whole list every week or so, while others just keep adding songs to their list for longer times.
We do notice that curators that update their list frequently actually generate way more active listeners.