The answer is -- it depends. It depends on what engineers call "supported joist length", load, and "allowable deflection". If the deck is narrow, the joists might be supported on the exterior wall at one end, and the beam at the other end. If the deck is very wide, there may be an intermediate beam, between the wall of the house and the outside beam. The intermediate beam would be doing twice the work of the outside beam, because it is supporting joists on both sides of it.
The supported joist length, is usually half the joist length on either side of the beam. Once the supported joist length is known, the total load can be calculated based on the weight per square foot that the deck will likely see. A load of 40 pounds per square foot would be typical however snow loads can increase dramatically if roofs are shoveled onto decks, or if snowdrifts are common because of prevailing winds.
Once the load is known, the spacing can be calculated based on acceptable deflection -- in other words how much are you willing to let the beam sag between supports. Having said all of that, the span will likely end up between 8 and 11 feet, however, many lumberyards and building supply stores offer free deck plans in the hopes that you will purchase the lumber there. Getting your hands on some plans, would be wise.