Soul Bound Doll familiar for a witch?

[Werebat]

A player is interested in playing a witch with a Soul Bound Doll familiar.

Seems reasonable to allow the witch PC to take the Improved Familiar feat in order to do this.

But at what level should the SBD become available? For example, the witch would have to be level 7 with the feat in order to take a mephit as a familiar.

Edit: Soul Bound Doll

[Tursic]

Level 7, A soul doll would be very useful.

[Ascalaphus]

There's a boon on a sheet in PFS that allows it at level 7. So that's a fair indication.