Sizable segments of China agriculture including rice, soybean, and corn have been moving away from urbanized southeast to much less developed northeast. The exception is cotton which has been moving to Xinjiang for plantation where produce much better yield than other places.
For remote and rural regions that are not suitable for living, the central government has subsided the local government and moved most of the residents to newly developed suburban. Those remote area would be mostly be functioned as national park or conservation and not suitable for farming.
As for those rural area that surrounding major cities in the north China, south China, and coastal regions, the central government has initially encouraged them to move to suburb with subsidies. However, even though most young people have moved to urban, there are still too many people still stay behind which mean that there are too much resistance to clear these area out for farming and other development.
To answer your question, it would take decades for these former farmers and rural towns to deteriorate to the extent that people are willing to take the government subsidized money and clear out the area for industrialized farming. It would be a slow and painful process as old people just don't want to move even though those rural towns don't have much of service or welfare.
For remote and rural regions that are not suitable for living, the central government has subsided the local government and moved most of the residents to newly developed suburban. Those remote area would be mostly be functioned as national park or conservation and not suitable for farming.
As for those rural area that surrounding major cities in the north China, south China, and coastal regions, the central government has initially encouraged them to move to suburb with subsidies. However, even though most young people have moved to urban, there are still too many people still stay behind which mean that there are too much resistance to clear these area out for farming and other development.
To answer your question, it would take decades for these former farmers and rural towns to deteriorate to the extent that people are willing to take the government subsidized money and clear out the area for industrialized farming. It would be a slow and painful process as old people just don't want to move even though those rural towns don't have much of service or welfare.