The answer would seem to be about two weeks. In Chapter XV, both Crispin and Joseph declare that it has been a fortnight. Moreover, in Chapter XXI (which takes place the same night that Crispin leaves), Crispin reminds Kenneth that their compact—a week before their arrival—was three weeks ago. This places the climactic evening of Crispin's departure on approximately September 25 or 26.
And yet it can't be quite that early. Chapter XIX begins thus: "When the Tavern Knight left the gates of Marleigh Park behind him on that wild October night…". Indeed, the last few days of the novel are placed in October no fewer than five times in that chapter and in chapters XXII, XXIV, and XV.
To do the least violence to the evidence I've placed the evening of Crispin's confrontation with the Ashburns on September 30. The reference to October in Chapter XXI is surely after midnight: fifteen minutes after he leaves, Joseph says that it's "past midnight" [Chapter XVIII]. So by then it's October 1.
The moon gives us justification and mild confirmation. October's new moon occurred the evening of the fourth. According to our chrononogy, this is the night that Cynthia ran away from Crispin, and "the night was moonless" [Chapter XXV]. Furthermore, there was "a crescent moon" [Chapter XIX] as Crispin left Marleigh for the first time, which is also consistent (except that the crescent moon would have risen closer to morning).