Starting later this year, the U.S. Marine Corps will guarantee all (well, nearly all) marines 14 months at their home base, for every seven month tour in a combat zone. The U.S. Army is planning a similar program (two months at home for each month "down range") by next year.
The U.S. Army has already increased the time troops spend at home to 14-15 months this year, taking advantage of all the troops returning from Iraq (and not being replaced there). This time at their home base, with their families, is called "dwell time," and the more of it you have between combat tours, the less likely you are to have stress related mental problems. More units are even getting 17-18 months of dwell time between combat tours. The goal is 24 months of dwell time. To that end, the army added 65,000 troops to its strength over the last few years, and developed software that went through everyone's personnel records to make sure everyone eligible to go overseas, went. That put less stress on the troops who seemed to be going over every 24 months.
In 2007, at the height of the Iraq fighting, army troops were spending a bit less than a year at home for every year overseas. Back then, the army was also in the midst of a reorganization, which didn't change the number of troops, or equipment, in a brigade, but did change how they are organized and used. The reorganization created more brigades, and made the army even better able to deal with the kind of heavy deployments required in 2005-7.
The math works like this. The army, marines and reserves can muster about sixty combat brigades. During 2004-7, there were 19 brigades deployed to combat zones (15 in Iraq, three in Afghanistan and one in South Korea.) That's when the army began working to get active duty troops two years dwell time for every year in a combat zone. For reserves, the goal was home for four years, overseas for one. It was believed that, with a little help from the marines, the army can just about make that. The increase in troops sent to Afghanistan will delay this dwell time plan for a few years.
There are several reasons for the two year dwell time goal. These include morale, keeping combat veterans in uniform, and the reduction of combat fatigue. The more you keep the troops in a combat zone, beyond a certain number of months, the less likely they are to re-enlist. Note that everyone in the army works on employment contracts (of 3-4 years, usually). Not everyone renews their contracts when they expire. But since September 11, 2001, an above average number of people have. That has gone up even more in the last year, because of the recession.
Keeping combat veterans in the army is very important, because people (officer or enlisted) who "re-up" are the most valuable people you can have in combat. But keep them out there too long, and they will start to leave. Not in large numbers at first, but eventually you will suffer large losses.
The U.S. Navy has had the same problem, because of the long deployments at sea sailors often had to endure. That experience enabled the navy to work out a formula, which calculated the number of sailors they would lose, for a taskforce, for each additional day, beyond the usual six months, they kept them at sea. The army is encountering a similar effect. The army is not publicizing their anticipated losses (people who don't re-enlist), but it was apparently up to several thousand troops a year. That doesn't break the army, but does provide more headaches for those in charge of recruiting and retention. The senior generals treat this sort of thing as "losses." Not combat losses, the people who don't re-enlist leave the army in one piece.
The more time you spend in combat, without dwell time, the more likely you are to develop combat fatigue. That can mean anything from transferring to a non-combat job, to a medical discharge (that gets you a pension and life-time medical care). Both of those options cost the army money. The army would rather see if additional dwell time will enable troops to recover from the stress the comes from being in a combat zone (even if not in combat). The additional dwell time would be possible if most troops were withdrawn from Iraq. That's not a sure thing until the Iraqis settle some of their current disputes.