How far can you span a 2 X 12 heade (2. 2' X 12" nailed together with a 1/2" plywood sandwiched between) it will be replacing an interior load bearing wall.
Single story house with rafters.

First, check with your local permitting Dept since they may have criteria. What type of wood for the header? Makes a big difference between spruce and SYP. In a 1 story home with only joist bearing and no other load 10-12 ft depending on wood.