I made a post previously covering "tours of duty" for U.S. soldiers during World War II here, but I'll reproduce it below and add some more information.

Being assigned to the Infantry branch (as roughly half of all men entering the Army were in mid-1944) was an assignment fraught with danger;

Deployed overseas|Total battle casualties|Deaths among battle casualties|KIA|DOW|Died while MIA|Died while POW|WIA|MIA|POW :--|:--|:--|:--|:--|:--|:--|:--|:--|:-- 757,712|661,059|142,962|117,641|19,613|1,795|3,913|471,376|15,830|56,212

There was no such thing as a "tour of duty" for U.S. soldiers during WWII; those on the front lines saw service until they were killed, wounded, taken prisoner, disabled, or otherwise too mentally broken to continue. It was estimated that a soldier began to lose his effectiveness after about 90 combat days and,was completely ineffective after 200-240 combat days. American troops were often on the front lines for as long as two months without a visit to a rear area rest center; many were not set up until October 1944 when the Allied advance was stopped at the German border. In contrast, the British had a policy of allowing four days of rest for every twelve days of combat. After 180 combat days, less than three percent of the original men in a unit remained, the rest lost to various causes. Neuropsychiatric casualties accounted for 15 to 25 percent of all non-battle losses (the Army hospitalized about 900,000 men for neuropsychiatric reasons during the war, a number Eisenhower demanded not be released to the press) and it was estimated that for every ten days of combat, three to ten percent of men in a unit became neuropsychiatric casualties. The "rate of replacement" for men varied wildly; many were killed nearly immediately, while others made it through the entire war without suffering any bodily harm whatsoever.

Men who showed particular aptitude for certain tasks were often sent back to the United States on temporary duty to attend specialist schools; they then returned to their units. Week-long passes were often offered to rear areas such as London or Paris. Combat veterans who had been made unsuitable for frontline service through wounds or other causes were often assigned to apply their expertise in training new soldiers at replacement training centers stateside.

In early 1945, to improve the morale of replacements (by now called "reinforcements"), General Joseph Stilwell proposed that they be pre-designated for assignment to certain units while still in the United States, and shipped in groups. Four men would form a squad, four squads a platoon, and four platoons a company. Companies and platoons would be broken up as needed, but the basic unit of four men would always remain intact. The plan was begrudgingly adopted by theater authorities in Europe in March 1945, and it is not possible to tell how effective it was since it only operated for a short time. The Surgeon General of the United States proposed that soldiers be given a six-month non-combat furlough after 200 to 240 days and have the option of serving it in the United States.

Before the invasion of Japan, it was proposed that infantry divisions be augmented with a fourth regiment (many being the "orphan" regiments detached from divisions when they moved from a "square" to "triangular" structure in 1940) so that one could be shifted completely out of the combat zone, as well as a policy that soldiers only serve 120 combat days before a substantial rest. As Japan surrendered, this was never implemented. A proposal was also made to rotate divisions completely out of the front line to rear areas, something done in the German Heer.

The replacement system in the Heer operated differently than in the U.S. Army, and it can be argued that there was an effect on morale. Divisions in the German Army were raised in certain geographic areas (not dissimilar to U.S. National Guard units) known as wehrkreise and men were assigned to divisional replacement units and took their training together, shipping to the front in groups, where they received more training before being assigned to a unit. The U.S. replacement system was rather impersonal. Men from all over the country (remember that the United States is massive; Germany is only about the size of Montana) were shipped to replacement training centers to receive instruction, and then shipped overseas to wherever they were needed, be it Europe or the Pacific. A port of embarkation (many replacements were assigned to augment divisions less than a month before they shipped out; the only significant small-unit training these men received would be combat) gave way to a trans-Atlantic or -Pacific voyage, intermediate and then field army replacement depots, replacement battalions, and then final units, each step of the way another obstacle to unit cohesion. Men often arrived scared and isolated, having little, if any, training upkeep offered along the way.

Men wounded in action who exceeded the theater's evacuation policy (usually 90-120 days) were returned to the U.S. Men whose wounds were severe enough to preclude overseas service, or service in the military in any capacity, were also returned. Fighter, photoreconnaissance, and bomber crews who completed a certain number of credited missions or flight hours were rotated home, but could also volunteer for a second tour in a different type of aircraft if they chose.

The military career of Audie Murphy, the most decorated soldier in U.S. military history, is surprisingly "average" discounting his heroic actions; many hundreds of thousands of widgets Infantry branch replacement troops walked the same shoes he did. Here is a brief timeline of Audie Murphy's military service, taken from his official records;

Date	Event
6/29/42	Made declaration and accepted for service at Greenville, TX
6/30/42	Voluntarily enlisted in the Army of the United States as a private from Dallas, TX, for a term of service of the duration of the war plus six months; attached unassigned to the Infantry Replacement Training Center, Camp Wolters, TX
7/7/42	Relieved of attachment
7/9/42	Attached unassigned to the 4th platoon, Company D, 59th Infantry Training Battalion, Camp Wolters, TX
8/3/42	Qualified with bayonet
8/15/42	Rated marksman with rifle
8/28/42	Rated unqualified with BAR
9/9/42	Rated unqualified with machine gun
10/13/42	Relieved of attachment; departed Camp Wolters, TX
10/18/42	Arrived Fort George G. Meade, MD
10/23/42	Assigned to Company K, 385th Infantry Regiment, 76th Infantry Division
12/9/42-12/14/42	6 day furlough
1/18/43	Relieved of assignment; departed Fort George G. Meade, MD
1/23/43	Arrived Camp Kilmer, NJ
2/8/1943	Departed New York Port of Embarkation
2/18/43	Arrived French Morocco
2/19/43	Attached unassigned to 2nd Replacement Depot, Fedhala, French Morocco
2/26/43	Relieved of attachment
3/3/43	Assigned to Company B, 15th Infantry Regiment, 3rd Infantry Division
3/21/43	Rated as rifleman, MOS 745
5/7/43	Promoted to private first class
7/7/43	Departed Bizerte, Tunisia
7/10/43	Arrived Licata, Sicily
7/14/43	Rated as messenger, MOS 675
7/15/43	Promoted to corporal
9/20/43	Departed Palermo, Sicily
9/21/43	Arrived Salerno, Italy
12/13/43	Promoted to sergeant
1/4/44	Rated as squad leader, MOS 653
1/13/44	Promoted to staff sergeant
8/4/44	MOS changed to platoon sergeant, 651
8/13/44	Departed Naples, Italy
8/15/44	Arrived southern France
9/15/44	Lightly wounded in action
10/13/44	Honorably discharged from the Army of the United States to accept a commission as a second lieutenant in the Army of the United States
10/14/44	Commissioned a second lieutenant in the Army of the United States; MOS changed to 1542, infantry unit commander
10/25/44	Lightly wounded in action
1/25/45	Lightly wounded in action
1/2?/45	MOS changed to 2900, company commander
2/16/45	Promoted to first lieutenant
3/10/45	Relieved of assignment
3/11/45	Assigned to Headquarters, 15th Infantry Regiment, 3rd Infantry Division
3/??/45	MOS changed to 1930, combat liaison officer
5/19/45	Relieved of assignment
5/20/45	Assigned to Company B, 1st Battalion, 15th Infantry Regiment, 3rd Infantry Division
6/10/45	Departed Salzburg, Austria; arrived Paris, France
6/11/45	Departed Paris, France; arrived Presque Isle, ME
6/12/45	Relieved of assignment; departed Presque Isle, ME; arrived Ellington Army Airfield, TX
6/13/45	Departed Ellington Army Airfield, TX; arrived San Antonio, TX; assigned to Headquarters, Army Ground and Service Forces Redistribution Station, Fort Sam Houston, TX
8/21/45	Appointed first lieutenant in the Officers' Reserve Corps
9/21/45	Discharged from the Officers' Reserve Corps at Fort Sam Houston, TX