The U.S. Army has established a policy, or policy goal actually, for how often troops can be sent overseas without their families. For active duty troops, it's once (for one year) every three years. For reservists, it's once every six years.
One of the additional problems with having an all-volunteer force is that most of the troops are married. That means that commanders have to take into account family needs. What the family needs is daddy, or mommy, or daddy AND mommy, home as much as possible. The war in Iraq required that many troops go over for a year, come back for a year (or less) and go back for another year. This was not popular with the families, although the troops themselves were a bit more understanding, as they were in the combat zone and could see what they were fighting for. Back home, the message was rather more mixed.
The new system fits in with the training and readiness cycle for units. Units spend their time in one of three states;
Recovery. When a unit comes back from overseas, or comes off a year or so being ready for action, there is a period of several months, or longer, when new personnel and equipment are brought in, existing personnel go on leave, or off to training courses, and the unit gets itself up to strength for a training program.
Training. With the unit up to strength in personnel and equipment, it trains together for up to a year, until it has passed all of its readiness tests, and is ready for another tour overseas, or standby status (staying at home, ready to go anywhere on short notice, with training to keep everyone in shape.) That is the third state; ready for action, especially overseas deployment.