GTA Online has shipped content for over a decade. The cadence is informative.
The pattern
Major content updates twice a year typically. Smaller seasonal events monthly. Specific patterns shifted over time - the studio leaned heavier into business updates in mid-life and into criminal-coded updates later.
Total content shipped: hundreds of vehicles, dozens of properties, multiple major heists, several map expansions.
What this implies for GTA 6
Probably similar cadence on the online side. Singleplayer content is harder to predict - GTA V didn't get story DLC despite years of speculation. Whether GTA 6 changes that pattern is unstated.