Is it possible for publishers to show popups for signup/offers using AMP-Lightbox component after a time delay (say avg time on site - 5s)? Any solution with amp-bind / amp-animation that could work?
This is not possible with amp-lightbox. The reason is that delayed popups are a bad user experience which goes against AMPs ideals. If you absolutely must do this, you can achieve this via CSS animations by fading-in an element after a certain delay.