If these are double trigger RSUs they don’t fully vest until two conditions are met:

1) the usual employed by the company for a certain period of time

2) some qualifying liquidity event (ipo / sale)

This is to avoid employees needing to pay taxes after #1 even though the stock can’t be sold. However for tax reasons there has to be time limit when they either vest or expire worthless. Depends on which lawyer you ask how long that can be, but generally most are less than ten years.