I don't like the word "regeneration" because regeneration is a process, while the transformation from a "heart of stone" (enemy of God / dead) to "heart of flesh" (drawn to the Son / alive) is an immediate transformation. The hot-mess that I was pre-Jesus was not 100% fixed in an instant, but something was instantly 100% different. That was my experiential reality.
So, the point in Acts 2 and Acts 16 that seems most important to me is not a focus on "was regeneration instantaneous" or "did it start at that exact instant" ... the important take away from both stories is "
GOD is the one that did something!"
- God pierced their hearts (they did not pierce their own hearts).
- God opened Lydia's heart (she did not open her own heart).
The thing that is important is not "ordo salutis" but "monergism, not synergism".