We make reservations for our family get together 9 months out because that's the earliest you can reserve a spot. Other CGs we go to are either 1yr or 6 mths out. We hardly ever book 6 mths out for ourselves. But we do live in Oregon and there seems to be an abundance of forest service CG that have spots open all year during the week. We will book a spot 3-4 mths out if a weekend is involved. St parks are a different story. Being retired and camping opens up more opportunities than if your a weekend warrior only.
My thinking is that it really depends on where and when you go.