It tends to vary by brigade (hopefully one our ROO community can chime in on this). My DS was a "walk on" non contracted Army cadet in 2015. By November of his freshman year, he was offered a 3 year. by the following February, his brigade extended it to 3.5 year including retroactive to January which was the beginning of his second semester.
But it is my understanding that some brigades and/or battalions prefer to offer more 2 year scholarships instead of extending half years. And I see the logic in that as well.
Best source of info would be your DS's own ROO.