In a recent blog from ZeniMax Online's studio director Matt Firor, he explained the Seasons are expected to run for "three or six" months, and will feature "a mix of themed story content, events, store items, dungeons and more." According to him, the move from yearly Chapters is easier on the studio.
Similar live-service games like Destiny 2 and Apex Legends have comparable three-month seasons. ZeniMax Online will gradually move toward making "smaller bite-sized content," in addition to "some larger items" that will release throughout the year.
"We'll be able to have teams launch content when it is ready throughout the year and not work to a date in June—this will let us focus on a greater variety of content spread over the year," he said. "This supports the new Seasons model, and will enable us to release content, updates, fixes, and systems in a more efficient manner."