
I have always started from day one.....well perhaps day 2 or 3. Can you start by leaving her in another room of the house preferably separated by a stair gate. (Well worth the investment). She will know you are around but it gets them used to not being worried by being separated from you. (Gives you a break from her too)
When you actually leave her, just 10 minutes or so to start with, make sure you don't make a fuss of her before going out, or when you return. Just treat it as a completely normal part of the day. When you come in don't go straight into the area she is in at first, walk around the other rooms for a bit. This will help her take it in her stride and not get over excited or anxious about your return.
Good luck with your new arrival
Sandra