I find that temperature affects it a lot more than the amount of snow. Even in deeper snow it's not so much power you need but the ability to maintain momentum. When you stall it's generally because you lose grip and thus speed also. You can't really ride a fatbike in any deeper snow than you can a regular one. The benefit comes from the wide tires that act as snowshoes and prevent you from sinking in while also providing a lot more grip compared to narrower tires. With good balance you can ride on snowy trails that are really difficult to walk on.

I'd say that in summer I can do a 45km trip of mixed single track and gravel road quite comfortably but in below 10 degrees celcius we're speaking more like 25km.