I don't know aboput other combatants. Germany pulled units back for reconstitution. US operated with a bare minimum of units and thus needed a large pipeline of trainees/hospital returnees. Divisions in ETO would "burn through" their infantry strenght in 90 days. The Army was trying to adhere to a 17 week training cycle for infantry replacements which would mean that you would need four trrops in the training cycle for every troop in a line infantry unit. Add in the over headfor running the infantry training centers and the infantry OCS plus the sick and wounded infantrymen at some point in the medical chain then the guys "in transit" to or from the front and you quickly get to a 1.5 to 1 ratio.
From the same book that I quoted above:Biennial Reports of the Chief of Staff, page199.
". . . At the replacement training centers men were made ready to join the divisions and replace casualties in a concentrated training period of 17 weeks. At these training centers they were given six weeks of basic military training and intense physical conditioning. In the remaining period they acquired competence in handling the weapons with which they would fight or the equipment with which they would work and in learning the tactics of squads, platoons, companies, and battalions, the tactical units which actually engaged in combat.
An infantryman, for example, became proficient in his primary weapons and familiarized with the M1 rifle, the carbine, the hand grenade, the rifle grenade, the automatic rifle, the .30 caliber medium machine gun, the 60mm mortar, and the two-man rocket launcher. These were the weapons that every infantry rifleman might be called upon to use. Not only were men taught to handle their weapons with proficiency in the replacement training centers, but they were taught to take care of themselves personally . . .As the Army acquired battle veterans, both officers and enlisted men were returned to the United States for duty as instructors in the replacement training centers. These veterans, who learned how to survive in combat, passed on knowledge to new men and thereby increased both their effectiveness and their chances of survival in their first experience in combat. . . "
Another thing is depending where the replacement was sent they got additional training and integration into a squad before being committed to combat. This was more true in the Pacific than in the ETO or MTO. Some divisions in the ETO ran new replacements through some training centers at a divisional school before sending them into combat.