If you are asking about paying the balance of a vacation package, you have to pay it off 45 days before arrival.
If you are asking about paying the balance of a room only reservation, you have to pay for it on arrival at the resort.
If you are asking about paying for charges that you make on your KTTW card, it depends. Each level of resort has a charging limit. I believe it is $500 at values, $1000 at moderates, and either $1500 or $2000 at deluxes. When charges on your KTTW card reach the limit your card is immediately charged with that amount. If you do not reach your limit, the card charges go on your credit card the night before you check out.
Occasionally you may reach this limit while in a park. They will refuse to allow further charges until the card has been paid off. Don't panic if this happens- just go to any Guest Services and they can take care of it for you. You can always ask at the front desk of your resort to find out how much you currently have charged to your card and have it paid off right away if you are close to the limit.
HTH