It did take hundreds of millions of years to form. When Earth first formed 4.5 billion years ago there were just a carbon dioxide and nitrogen atmosphere. Then about 3.5 billion years ago the earliest oxygen producing microorganisms came about and we started getting a trickle of oxygen in the atmosphere generated by combining water, carbon dioxide and sunlight. But levels were still low until about 850 million years ago when the oxygen levels in the atmosphere shot up and most of the carbon dioxide was converted to oxygen and organisms. This was right about when the first plants developed, and we started getting multicellular organisms. Since then the plants have been able to maintain current oxygen levels. Or at least somewhere between 15% and 35%. Without plants we would not have oxygen.