How long is she expected to continue studying abroad ... 1 year? 2 years? 3 years or more?
If she's going to be traveling like that for 2 years or more she should get a reentry permit. Even though having a reentry permit isn't absolutely necessary if she's going back to the US every 5 months, a long pattern of traveling like that without a reentry permit could create problems at the port of entry.
You mentioned you're a US citizen ... do you work for a US government agency or US corporation, or a recognized international organization to which the US belongs such as IMF, NATO, UN? If yes, and you will be stationed abroad for at least a year, your wife may be eligible for expedited citizenship under section 319(b).
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.