Split time between US and Japan while keeping Green Card

My wife is a Japanese national and US green card holder (25 yrs). We are considering moving back for at least part time residence and curious if anyone with the same immigration status has experience splitting time between the two countries (e.g. 6 months of the year in each) and whether they have had any hassles when returning to the US? Online sources suggest a 1-year cliff before they start asking questions, but I also understand that it can be a gray area, and at the discretion of the border officer.