"When" is inevitable but that could be 40 years and 150,000 miles from now. Yes, aluminum will eventually fail from fatigue but it's practical lifetime is extremely long unless subject to absurd abuse. I have a 1992 Trek 1420 still in routine use by my son that shows no signs of giving up. Time frame? Decades.

Trek's (and all other makers) "lifetime" warranty is based on experience that assumes that very few of the frames will be ridden enough to fail either because 1)the original owner moves on to something else in a few years, 2)some bikes are ridden very little and the put away for good or 3) the frames won't fail at any appreciable rate even if ridden routinely for many years.