Answer:
The mother Possum is pregnant on average 13 days. Then the young possums will stay in the mother's pouch for about 2 months to 7 weeks. After the young have spent all that time nursing inside the pouch, they have become covered in fur, and gotten bigger. They will now venture out of her pouch, they are still very small, about 2 inches long excluding their tails. The mother will carry her young everywhere, they will cling to her fur on her back and sides, until she is ready to wean them. For the first couple of weeks they will continue to nurse, after 2 weeks they are usually ready to eat adult food, and about a month after they start eating an adult diet, they will be fully weaned. For more details, please see the sites listed below.
The above refers to the Virginia Opossum, but not to true possums, which are native to Australia, and quite unrelated to opossums. There are many varieties of possums, with different gestation periods, but they tend to average between 16 and 18 days. Because there are so many varieties, and thus so many sizes, the amount of time they stay in the pouch varies.
Most smaller species such as the six species of gliders stay in the pouch for about 70 days. The smallest pygmy possum, the Little Pygmy possum, only stays in the pouch for around six weeks. The much larger Brushtail possum stays in its mother's pouch for around five months.