At the end of the novel, Rhett states he is 45 in 1873, which means he was born in late 1827 or sometime in 1828. Based on that, he probably started in at West Point in 1844. West Point is a four year institution and when he served in the Confederate Army, Rhett was placed in the artillery. Due to his military knowledge, he probably finished at least 2 or more years of his education. "Everyone knew in detail how he had been expelled from West Point for drunkenness and 'something about women'. That terrific scandal concerning the Charleston girl he had compromised and the brother he had killed was public property. . . . that his father, a charming old gentleman with an iron will and a ramrod for a backbone, had cast him out without a penny when he was twenty and even stricken his name from the family Bible. After that he had wandered to California in the gold rush of 1849 and thence to South America and Cuba, and reports of his activities were none to savory" (p.153). There seemed to be sometime from his expulsion from the academy to his expulsion from his home.
For more information on antebellum West Point History Net has a good article, Life at West Point of Future Professional American Civil War Officers that is not very long, but describes life as a plebe, courses of studies, and the fact the Academy mostly focused on turning out engineers and scientist to help build the nation.
Also, How Do We Run On: A Gone With The Wind Scrapbook has an excellent post on West Point education in the 1840's, and a wonderful comment on real students who were expelled-Edgar Allan Poe being one of those students.
