The faster you travel wind noise will often shroud noises that the vehicle is making so don't think that the fault has gone away at that point.
It depends really on how bad the bearing is with regards to what driving it's ok to continue with, really a failed bearing should be replaced ASAP. Some bearings will just hum along for months and months.. But that hum is coming from a damaged/pitted bearing surface, and nobody knows for sure how long they'll really last before the bearings actually starts to break-up/crack etc.
Wheels won't normally come off due to a failed wheel bearing, but they can in some circumstances.. Say an outer bearing race collapses and wears away the stub axle washer & nut, then a wheel may come off.
It doesn't really matter what roads your driving on as a failed bearing, especially a bad or noisy one will start to generate its own heat and self destructive environment, so even driving reasonably slowly won't stop it wearing.