The maximum span for a 2 X 10 floor joist is up to 21 feet, depending on several factors. One must take into consideration the spacing of the joists and type of wood used. Additionally, the load that the floor needs to support is also a consideration.
A Douglas Fir typically has the ability to span a larger distance than a Spruce Pine or Southern Pine. In situations calling for a greater amount of pounds per square foot capability, the span length will decrease. A first floor joist also typically requires a higher psf rating than a second story or attic.