My DD2 is 14 wks also and I still do a DF at around 10 as well. With DD1, we did it until she was regularly STTN (around 6 months). I think the conventional wisdom is that you drop it when they are STTN, or whenever it seems to make no difference in when they wake up. The only way you can really tell is stop the DF for a few days, and if he still wakes at around 1:30, then you know the DF is not helping and you might as well discontinue.
