The question of scarf ratio often comes up in discussions. I did some math and concluded the minimum ratio was a hair longer than 3:1 for typical woods, assuming the glue bond is as strong as the wood. I don't recall the precise formula but it is roughly sqrt(A/B)) where A is the tensile strength of the wood along the grain and B is ditto across the grain.

I made and tested several 4:1 scarf jonts which did not fail when the test pieces were broken across the joint. I had a 4:1 joint fail on a bend; the glue was starved and it has held up since being reglued. However, such short ratios can have problems:

demands greater cutting acuracy and care when gluing
water penetration into the end of the joint can start separation on a sharp bend
gluing is more difficult as the pieces tend to slide apart when clamped
glue can be absorbed by woods with open grain cut at a coarse angle
some woods are harder to glue well (oily, dense grain)
there is no safety margin

So 6:1 is a practical minimum and as Bill says, use a longer joint on a bend. I would go 8:1 underwater on a boat used for long trips.

The longer scarfs recommended in old literature (12:1, 16:1 or even 20:1) predate the advent of modern glues and are unnecessary these days as well as wasteful of material.