The fatigue strength is way below steel. So most aluminum frames are designed with factor built in. i.e. thicker tubes walls than needed. Than has aluminum fatigues for a while frame is less stiff and near what frame should be in terms of ride quality. But after a lot of miles an aluminum frame will become flexible beyond what is desirable for good riding frame. A correct designed steel lugged frames should be good for at least 25,0000 miles. An aluminum should peak out way below that mileage.
And Aluminum frames should be off topic so reply off line
Walnut Creek, CA