I recently made a post about the psychological hazards of serving in the Infantry branch of the U.S. Army during World War II that also dealt with length of combat service before relief;

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 of these centers were not even 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 the 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.

Sources:

• Atkinson, Rick. The Guns at Last Light: The War in Western Europe 1944-1945. New York: Picador, 2013.

• Ruppenthal, Roland G. United States Army in World War II, European Theater of Operations, Logistical Support of the Armies Volume II: September 1944-May 1945. Washington: United States Army Center of Military History, 1959.

• United States. United States Army Adjutant General's Corps. Army Battle Casualties and Nonbattle Deaths in World War II Final Report, 7 December 1941-31 December 1946. Washington: Statistical and Accounting Branch, Office of the Adjutant General, 1953.

• United States, United States Army Medical Department. Neuropsychiatry in World War II Volume I, Zone of Interior. Washington: Office of the Surgeon General, Department of the Army, 1973.

• United States, War Department, Army Ground Forces Historical Study No. 7: The Provision of Enlisted Replacements. Washington: Army Ground Forces Historical Section, 1946.

U.S. airborne divisions during World War II, particularly those serving in areas with favorable terrain or weather that made frequent parachute jumps a possibility, were not usually used after the fact in operations as conventional infantry long if the next operation was particularly critical or required extensive planning, instead rotating back to friendly areas far to the rear (in the case of the aftermath of D-Day and the month that followed, England) to train, re-equip, and absorb replacements. This is not to say that the U.S. didn't often use their airborne divisions in a manner similar to conventional infantry, or keep them at the front lines for an extended period of time. Regular infantry divisions, once committed, would never actually stray very far from the front lines except under extraordinary circumstances. Regiments within a division, battalions within a regiment, and companies within a battalion (usually a maximum of one of each at a time, if that) could be shifted into reserve to re-equip, reorganize, or train, but were never very far from the front lines.

The average amount of relief that an individual man in a selected infantry company at a certain time could expect was consequently quite short due to the stress of cumulative days of combat and the overall risk of becoming some type of casualty, but the amount of time the selected company as an organization spent in battalion reserve, regimental reserve, division reserve, corps reserve, or was otherwise not engaged, would not be insignificant when compared to the time the company spent in actual combat during its total period of service.