Once level 80, at the bottom of the AA window there are two radio buttons. One of them says something along the lines of "Time training". You click that button, and then all AA's in the window should light up as being possible to train. Click the one you want to train over time and a timer will appear over it. When the timer runs out, you get the next level of that AA. You then can select Time Training again, select a new AA feat, and continue.
If you cancel a feat being time trained, you lose all time put into it.
...or you can just do your math and calculate if it is worth it to spend points to finish of the timer. I think 18 hours equal 1 point. So if a timer ist at 35 hours i would spend 2 points to finish it, and start to train something else. If the timer is at 20 hours I would not spend 2 points, of course. There is no time wasted.
Also, the toon has to be 80 to be able to time train.