Most people on this board make them as early as possible. 6 months or so. The more popular restaurants book up very quickly, i.e. Cinderellas Table Breakfast.
I wouldn't make them the day before, you may not get your requested seating time.
That being said, I was at Poly last weekend. I called at 8:00 p.m. for an ADR at 'Ohana, and was able to get a table for 6 at 8:30 p.m. It never hurts to try, but if you have your heart set on certain restaurants, the earlier the better.