When did oxygen levels start to decline from 35 percent down to today's levels during what period

During Carboniferous period, part of the late Paleozoic era.

Explanation:

In the geological story oxygen levels have been relatively stable for the las 200 million of years, but 300 million ago there was a dramatic spike in Oxygen levels until 35% in the Carboniferous, next it plunged in 15 percent in the Triassic, 100 million years later. The growth of the forests removed huge amounts of carbon dioxide from the atmosphere, leading to a surplus of oxygen. The higher oxygen concentration is related with the presence of larger bodies in reptile and insects.

about 95 million years ago, during the Carboniferous Period