In Maggie Dresden's Timeline it says...

+ It is unclear when she sets up the special Harry and Thomas soul gaze message.

...however based on biology and info from PG I think we could accurately place this withins weeks of Harry's birth.

Reasons:

1) The Obvious - Harry had to "exist" for the spell to lock to him.  So at minimum it would have to have occurred within 8 months of his birth.
2) Baby Hair - In PG, Harry fails to find Molly using her baby hair.  Bob states that this is because babies are mailable and Molly is no longer what she was  then.   If you extend that concept to a fetus, well, anyone who has seen a sonogram knows that an early pregnancy fetus does not much resemble a baby.  The changes during this period are radical therefore it would make sense to wait as long as possible for that fetus to be very near it's completed development prior to trying to imprint it. 

So in my book this would place the casting of the spell in the Third Trimester minimum, I think one could safely say October.