If the atoms that are bonding have identical electronegativities, then it's a completely nonpolar covalent bond. This doesn't happen in the real world unless the two atoms are of the same element. In a practical sense, any two elements with an electronegativity difference less than 0.3 is considered to be nonpolar covalent.
As the difference between the atoms increases, the covalent bond becomes increasingly polar. At a polarity difference of 1.7 (this changes depending on who you ask) we consider it no longer to be a covalent bond and to be the electrostatic interactions characteristic in an ionic compound.
Just so you know, you shouldn't take these values as exact. ALL interactions between adjacent atoms involve some sharing of electrons, no matter how big the difference in electronegativity. Sure, you wouldn't expect much sharing in KF, but there's a little sharing of electrons anyway. There's certainly no big cutoff that happens at a difference of 1.7 Pauling Electronegativity units.
