1. What type of load is being supported? Is this a floor and if so what is the use of the room (storage, sleeping, living)? Is this a ceiling and if so what is above (floored or unfloored attic, another room)? Is this a roof and if so do you live in an area where snowfall occurs (even rarely)?
2. What building code governs at the site of construction? Standard, Uniform, BOCA, IBC?
3. What is the spacing to the next repetitive member?
4. What is the finish material on thewood member? None, gypsum board, plaster, metal, masonry?
5. Are there any expected or possible unusual loads to be placed on the member? Safe, book storage, waterbed, etc.?
6. Is the loading temporary or permanent.
As you can see the simple answer you seek involves a lot of questions and considerations. Beam spans (and I assume you are using this as a beam or joist) are limited by deflection (amount of allowable sag), compression or bearing failure, or by shear failure (tearing of the wood) and include a built-in safety factor that allows for defective materials and unexpected loads. Bracing to prevent rollover is also a consideration. Check with your local building offical. He may have a simple chart that considers the worst case senarios and shows maximum spans he will allow. Some codebooks include such charts. Otherwise, an engineer or architect can custom calculate the conditions and may certify a greater span but the cost is usually not reasonable for small projects.