
Commend you for being concerned about your daughter.
I was a child who the parents didn't tell ANYTHING; Mom just took off, no warning whatsoever.
So, I vote for a week. That way she is forewarned but not so long before that she has so long to fret too much.
I'd let her feel like she had some control about where she wants to split her time. At 9-10 let her be as flexible as SHE needs; no hard and fast rules. Don't be jealous and competitive over her choice of who she will pick as a primary parent.
Kids, IF allowed to do so, will migrate to the parent they feel they need most AT that point in time; it will change back and forth and you two need to be flexible and leave out anything that would make her feel guilt toward her choices.
Example: "Mom, I want to stay at Dads more than four days a week..I want to stay for two weeks..."
Mom: "Sure! Dad loves you as much as me, so I think he will be very happy with that!".
Don't make her visitation so tied to the $$$ of child support that arguments come up over "Well, I fixed her 7 meals last week and you only had her twice, so you owe me another $75.00 because that is 62.4percent of blah....blah...blah..."
You would not BELIEVE the things I've heard in mediations.