Even when we get to the Dominion War the Federation has developed a policy of rotating ground forces off the front after only 3 months (while for example with the war in Iraq and Afghanistan today, rotation is a year. And World War 2 was the duration, with some people being in war for years).
Hmm, DS9 AFAIK speaks of rotation from the actual fighting in front lines. Which in WWII might have been in the order of two weeks or so on the average, three months being quite an exception... Doesn't mean these folks would have gone back home; they'd just become fighting reserves instead of fighting frontline units.