Onion types run the gamut from larger red and sweet onions to green or white scallions. As a result, this means that size and maturity times can vary with each species. This may range from 60 to 100 days for onion bulbs to mature. Nevertheless, all onions should be planted about six to 12 inches apart in a sunny area with alkaline soil. When harvested, they can last for a month in your refrigerator.