According to DowningPumpkins.com, pumpkins should only be carved within a day of use. However, bleach and vegetable oil can extend the life of a carved pumpkin.
According to About.com, soaking carved pumpkins in 1 gallon of water and 1 teaspoon of bleach for a couple of hours helps harden the pumpkin and extend its life. After soaking the pumpkin, dry it thoroughly with a clean rag. Rub the entire outside, inside and carved areas of the pumpkin down with vegetable oil to prevent shriveling. Petroleum jelly can be used in place of vegetable oil. Repeat the soaking, drying and oiling process for any pumpkins that start to shrivel.