I think Brian Hicks officially said recently that it is 8 days, moving items is supposed to refresh that timer.
However…
In practice, it appears that Hicks’ information is outdated or too soon, some base items haven’t been touched in over 8 days and are still persistent. Plus, we are finding that paper, garden plots, and other junk is staying on the ground.
Excess junk is causing our server to crash, and we aren’t the only ones. DayZ Underground is also crashing probably due to excess junk from how loot is working right now.
To be safe, just keep refreshing your stuff at least every 8 days and you should be good.