Wheel shaking

Possible bent wheel, tire out of balance, worn front suspension or bad wheel bearings.

The most common thing is the tire is out of balance, it may have lost a weight and the problem started, or like the answer above, you may have hit something bent the wheel and lost a weight. Another possible problem is worn steering components, if the wheel starts shaking violently, you probably have play in the front components....the tire get deflected and starts shaking. Sometimes you can jack up the wheel and feel the slop by moving the tire with your hands, in general, you need an alignment shop to inspect the car, there is no set mileage when these thing wear out as the wear is directly associated with the type of roads the car is used on, If you live where the roads are bad, that will wear the front components faster.