That's an excellent question! It's not clear exactly how long, but it could have been as much as 2 to 5 years, based on the timeline established in David's journal.
- 1999 - Keira and Nora move in with Hunter, Annabelle, & Drew.
- 2002 - Annabelle died.
- 2004 - Day after Catherine's 3rd birthday, David walks in on Nora and Keira fucking Drew, then Hunter.
So it depends how fast Drew and Hunter put the moves on Keira and Nora. If Hunter started after Annabelle's death (as he claims) then he could've been fucking them for 2 years. If Drew started shortly after they moved in together, then he could've been fucking them for 5 years!
EDIT:
It may have
started 21 years ago (1999, as PV takes place in 2019), but Ophelia's kids were all born by the time David found out about Hunter/Drew/Nora/Keira 15 years ago (2004).
EDIT 2:
It's also mentioned that Lucia was born 2 months after Sterling, and 11 months after Keira moved. If Drew or Hunter is Lucia's father, that would mean they got Keira pregnant only 2 months after she moved in!