Welcome.
Manufacturers and owners of ebikes make some fantastic claims. I'd say that it might go 100kms on a charge if you pedalled 90% of the time and used the electrics 10%. The Chinese bikes that look like scooters are very difficult to pedal, so I imagine you would use electrics 90% of the time, so at a guess without much in the way of hills or headwinds you might get 20 kms+.
You can get a hub conversion to fit a mountain bike, but usually they are for the front wheel and suspensions and front hubs don't mix.