Is it highly recommended to apply for the AOS (Adjustment of Status) before the I-94 expires?
Certainly, a significant number of people are applying after the I-94 date has passed. In fact, perhaps most, simply because typically it's only a 90 day stay and it takes longer than that to ever get through any immigration process. So, it's quite common that people are applying after their I-94 exit date.
That, in itself, is not a problem. Now is it eight years after your exit date? Even that is not always a problem (when related to spouses). We've seen people come on a tourist visa, five, six years later they marry a US citizen and now they want to do an adjustment of status. These frequently get approved. So, speaking normally, in normal circumstances, it's absolutely not a problem if it's expired. But your specific question is, should you apply before it's expired? The answer really comes down to what kind of petition you're on. If you came on a K-1 visa, yeah, absolutely. As soon as you're ready, go ahead and apply.
There's not a strict deadline. But you're going to be stuck here, you won't be able to travel or work until you do file. So, we would recommend you do it right away.