What happens when you leave a pituitary tumor untreated?

Most tumors grow. Even benign pituitary tumors may grow, albeit slowly. Observation may be appropriate for older or very ill patients, but treatment is important to establish a diagnosis, and smaller tumors are easier to remove without complications. Most of us collect medical problems, making surgery safer now than later in life. Untreated tumors may cause vision problems, headache, and hormone abnormalities.
Depends: size & type. It depends on what type of pituitary tumor and how quickly it grows. Some start small and grow slowly so don't cause symptoms for years. Large or fast growing pituitary tumors can cause loss of peripheral vision, headaches, and/or a variety of symptoms from endocrine (hormonal) imbalances which depend on histologic type.
Pituitary tumors. May make hormones, like growth hormone, which produce acromegaly, but the location is adjacent to the optic chiasm, so visula abnormalities are possible. A lot depends on the symptoms. Usually these grow slowly.