How long must an EB-5 green card holder live in the United States each year?

There is no minimum amount of time required by U.S. immigration law and regulations for a lawful permanent resident to be physically present in the United States each year. In order to maintain the green card, the person must "intend" to permanently reside in the United States. If the person stays out of the United States for more than six months on any given trip, it raises a rebuttable presumption that he or she "intended" to abandon the green card. If the absence is longer than one year, the intention is assumed and it is pretty hard to rebut it (you need a really good reason, like an extended hospital stay or other extenuating circumstances). Wanting to get medical insurance in the United States is probably not enough of a reason (or not a valid reason) for a person to immigrate to this country if he or she is not planning on actually living here. If their intention is to reside here, they can spend more time outside of the United States by applying for and obtaining reentry permits, which are valid for two years and can be used to enter the United States at any time, regardless of how long their absence has been (i.e., less than two years) and they meet all of the other requirements for entry.

Lawful permanent residents who obtain their "green card" through the EB-5 program have the same residency requirements as any other LPR. Lengthy absences can lead to abandonment and loss of permanent resident status.

That would likely put their green card at risk. Permanent residency really means permanent.

The rule is that the holder of a green card is supposed to live in the United States to successfully retain their green card. Also, based on your plan, it appears your goal(s) for seeking EB-5 green cards is fraught with the possibilities that you will eventually lose those green cards, even if you obtain them, if your parents will not live in the United States for more than 2-3 months per year. Advisably, before you pursue your plan, seek counseling with an EB-5 attorney because there may be other visa alternatives.

Not easy, but if you had a residence in the United States rather than, say, a hotel, it might help. A letter from your attorney stating that they need to receive different treatments in each country might help if they are questioned at the Port of Entry.

They must have an intent to live here permanently, but absences can be excused provided you obtain advance permission via applying for reentry permits.

In response to your question, once one is granted Lawful Permanent Residence in the United States, he cannot "abandon" his residence. One's residence is presumptively deemed abandoned, if he or she remains outside of the United States for a period in excess of six months (continuously). Moreover, your parents would have to be the direct investors in the EB-5 project, and the project must go on to meet the job creation requirements.

If a lawful permanent resident (i.e., a "green card holder") spends greater than 12 months outside the United States without an intervening trip(s) to the United States, then there is an assumption of abandonment of permanent residence. In order to prevent this from occurring, you may file for a travel permit declaring residence in the United States, but plans for extended stays abroad to preserve permanent residence.