That depends on cost and value. For me, I can be profitable after an average of twelve clicks per page, lifetime. It is virtally impossible to think of a bingo activity too obscure for me.

Anyhow, guess and check. If you write and cannot rank for competiton, go down tail. If you write and cannot profitably exploit traffic trickle, move upwards.

Note that entire portfolio of content gets better over time if you do it right. (Superlinear returns on linear cost.)