It all depends upon where and when.
We live in Indiana, so we make reservations for Christmas week in Florida or South Carolina in January.
We make reservations in March (Florida or South Carolina) in April or May the year before.
In Indiana, for dates like July 4, Memorial Day, Labor Day, we make those reservations on or as near as possible as Jan 1, as in Indiana you can't make reservations until January 1.
Everywhere else, we may make reservations 2 weeks out (If doing State Parks in Indiana, Illinois, Ohio, Michigan, or Kentucky. Or if we have a known date we want to take, we do it immediately, as soon as we know we have the time off work.
The days of just showing up and getting an open campsite are pretty much over for everything East of the Mississippi River.
We've been fortunate too. In 31 years of marriage, we've had to cancel a campsite only once. We've canceled 2 air-line flights and a Carnival Cruise, but only once in 31 years have we canceled a campsite!
We plan ahead, pay ahead, and enjoy.