It's pretty common I think (in Russia too). Can't say the exact fugures but Ukrainian yellow pages gave me 722 results (=families, not persons).
is it a ruthenian surname?
May or may not belong to a Ruthenian. I don't think one can always tell the nationality basing on the last name only. My guess is that it could belong to families that had close ties with Western Europe (possibly from Western Areas of the modern Ukraine), hence the base "leon"="lion" which is not Slavic. In Russia it turned to a very common last name "Leonov".