Probably the same as the minimum visibility for takeoff or landing. After all, why taxi unless you're going from the ramp to the runway or vice-versa?
If you can't see well enough to find your way, don't taxi! While SMGCS will help in Cat II and Cat III weather, you may find that pockets of thicker fog exist, so you can't see ANYTHING! Been there; done that (in SCL a few years ago, where a fog bank was rolling in, and reached midfield when we landed; waited for a follow-me after clearing the runway).