Frank,
They were called to service based on President Lincoln's first call for troops. It was thot that the war would not last longer than three months. We read about 100 day regiments, but, I think they mean 90 day regiments. Illinois's first six regiments were 90 day regiments. I think that the 11th Infantry Regiment was the last one. Starting with the 12th, they were three year regiments.
Frank
Bye the way, some of the 90day units stayed on until they were replaced.