As of 2019, the month has just been to Sanya duty-free store, has left the island can go back to the duty-free store again to buy again, there is no restriction on the number of days to leave the island, but rather on the total amount of shopping now, the relevant state regulations are as follows:
"Announcement on Further Adjustment of Duty-Free Shopping Policies for Hainan Outlying Island Travelers"
I. Will the outlying islands travelers (including the island's resident travelers) per person per year The cumulative duty-free shopping limit for each person (including island resident travelers) is increased to 30,000 yuan, with no limit on the number of purchases.
Secondly, some household medical equipment commodities have been added to the list of duty-free commodities on the outlying islands, including vision trainers, hearing aids, orthopedic fixation devices and household respiratory support equipment (non-life support), with a limit of two pieces per person per purchase.
The tax-free policy for the islands applies to domestic and foreign travelers, including residents of Hainan Province (i.e., island residents), who are at least 18 years old and leave Hainan's main island by air but do not leave the country.
The policy was downgraded to 16 years old on October 24, 2012
Expanded information:
The following conditions must be met simultaneously for duty-free shopping for outlying-island travelers:
Firstly, having purchased a ticket for the outlying-island flights and holding a valid identity document, domestic travelers (including Hong Kong, Macao and Taiwan travelers with valid travel documents), and foreign travelers with passports;
Secondly, they should pay for the duty-free commodities in the designated outlying island duty-free stores, and the varieties of commodities and the number of times, amounts and quantities of the duty-free purchases should be within the scope of the national regulations, and they should obtain the shopping vouchers according to the regulations;
Thirdly, they should collect the purchased duty-free commodities with the identification documents and shopping vouchers at the airport segregation area and pick up the purchased duty-free commodities at the designated Thirdly, the purchased duty-free goods shall be withdrawn at the designated pick-up point in the isolated area of the airport with the identity card and shopping vouchers, and shall be carried by the travelers themselves on the airplane to leave the island.
Central People's Government - Announcement on Further Adjustment of Duty-Free Shopping Policy for Hainan Outlying Island Passengers
Baidu Encyclopedia - Duty-Free Policy for Outlying Islands