You need to comeback within one year, provided your abroad stay is temporary in nature. However, if you are living outside for multiple years, just coming back to USA once in a year may not be good enough to keep your GC. USCIS website has pretty good guideline for how to maintain PR status. If you plan to stay outside more than a year (ignoring the fact that you are coming to USA once in a year), it's better to apply for re-entry permit (re-entry permit is good for two years).
