According to Gardening Know How, the first step to ripen a green pumpkin is to remove the pumpkin from the vine, leaving at least four inches of vine at the top. Then, the pumpkin needs to be cleaned with a diluted bleach solution to prevent rot and mold. Finally, the pumpkin should be placed in a warm, dry and sunny spot with the green side facing the sun.
In order to limit the pumpkin's growth and divert the plant's energy into maturing the existing pumpkins, About.com recommends pinching back the vine tips when the fruits first appear, although this practice might minimize the production of additional fruit. Pumpkins need plenty of sun for their fruit to fully mature. An average growing time for a pumpkin is between 90 and 110 days. Gardeners who live in a region with a short summer season should make sure to buy varieties that are likely to mature before the sunny season ends.
Some pumpkins are fully ripe even though they are still green. According to Gardening Know How, a ripe pumpkin has a hard stem, and it has firm skin that does not puncture when pricked with a fingernail. It also sounds hollow when thumped.