the work you get at every vertical is different. The work of an SA (and responsibility) will take time to learn and perfect even with 4 years under your belt. Similar change when you become PA.
All in all in 3-4 years you can learn the entire life cycle of transaction fairly well.
Tier 1 firms are involve less vetting and more diligence where the former is kept for the senior rung of the people in your team. Due to this, it will take atleast 3-4 years to get a sound hold on commercials as well as their varied nuances. At Tier 2s and boutique firms, one's vetting exposure is higher along with due diligence owing to which 1 or 2 years is enough to absorb the fundamentals of M&A/PE/VC dealmaking.
How many years into the profession from your A0 days did you get the hang of 99% of the required knowledge? I am talking about a point where you needed absolutely no instructions from your senior and you simply knew what had to be done from A to Z. Thanks.
All in all in 3-4 years you can learn the entire life cycle of transaction fairly well.