How far can a 2x8 free span?

It varies depending on what you are spanning and what kind of wood you use. If you are referring to floor joists , here's the numbers for a few wood species. This is for joists at 16" on center.

Southern Yellow Pine #1 Grade 9'-11"

Southern Yellow Pine #2 Grade 9'-9"

West Coast Douglas Fir #1 9'-11"

West Coast Douglas Fir #2 9'-9

Hemlock Fir #1 Grade 9'-6"

Hemlock Fir #2 Grade 9'-1"

Spruce/Pine/Fir Southern #1 8'-10"

Spruce/Pine/Fir Southern #2 8'-7"