You may leave the US for as long as you wish - in exactly as a US Citizen can do. However, you are required to do 2 things:
1) Apply in advance for a Re-entry permit. This will allow you to be away for more than 6 months consecutive. Last time I looked these were about $750 to apply for and one is required for each family member.
2) You are required to file annual tax returns to the IRS regardless of where the money was earned- exactly like a US citizen.
To file taxes requires a good Tax Accountant who is familiar with both US tax and the foreign country tax rules. They have to handle the the Dual taxation between the US and your temporary country. It is complex and can be expensive.