Fuel dumping is common on both large commercial widebody airliners and virtually all military planes. Many aircraft require the ability to dump fuel because of landing weight restrictions imposed by the manufacturers. Although a plane may be able to takeoff at a certain weight, its structure and landing gear may only be able to withstand the impact of landing at a much lower weight. This difference between maximum takeoff weight and maximum landing weight may be tens of thousands of pounds or even over a hundred thousand pounds for very large planes. In other situations, an aircraft may be at an acceptable landing weight but forced to make an emergency landing at a runway that is too short. It may be necessary to dump fuel to reduce the plane’s weight low enough in order to make a safe landing on a shorter field.

It is certainly possible for a plane to land safely when heavier than its maximum certified landing weight. This weight is based on more than just the structural limitations of the plane but takes factors like runway length, center of gravity limits, and flap deflections into account. Airline pilots are often forced to land at weights above the certified limits because of bad weather, scheduling, or fuel conservation requirements. While such landings pose no safety danger to the passengers, a maintenance inspection is required any time a plane lands above its certified landing weight. The length and complexity of this check can vary depending on the quality of the landing. If it is smooth and uneventful, the inspection may be brief. A hard landing, on the other hand, might require a time-consuming and maintenance-intensive checkout that takes the plane out of service for a considerable length of time.
- http://www.aerospaceweb.org/question/planes/q0245b.shtml

TIME TO DUMP FORMULA:
http://www.atpflightschool.com/students/downloads/PrepFormulas.pdf
2,300 lb. / min was given as an example

As an example of fuel dumping…
Anytime a plane takes off and has any sort of issue right after takeoff it has to return and dump.

“Residents in the vicinity of Newark International Airport were complaining about the smell of gas Sunday afternoon after a Boeing 777 dumped 170,000 pounds of jet fuel into the air while preparing for an emergency landing. The plane, carrying 269 passengers bound for Tokyo, returned to the airport less than an hour after takeoff yesterday because of a “hydraulic problem.” But before it could touch down, it needed to lighten up by emptying its fuel tanks over the Garden State.”
- http://gothamist.com/2010/05/10/lots_of_jet_fuel_dumped_over_jersey...

Aircraft Pollution in tons per year over NYC alone.
Landing & Takeoffs (LTOs) 383,206 452,950
Volatile Organic Compounds 3,025 4,839
Carbon Monoxide 8,712 12,808
Nitrogen Oxides 5,729 9,574
Sulfur Dioxide 270 364