Two of my gearboxes have done this and both times its been the main shaft bearing that has failed I dint drve with either box for long, as soon as i herd it was on its way out i got it back to work (did no more than 40 miles). Once the bearing has filed the main shaft stars to move about, this causes the gears not to mesh correctly which results in the teeth being damaged.

It impossible to say how long the box will last!