Minecraft L2D resets every four hours. The only exceptions are when the server's TPS(ticks per second) is down, or it is 12am EST, those are the times the server will restart. The resets are when the zombie maps get reset to restore loot and the buildings. Plotworld is not effected by the resets/restarts, as such you should build and store loot in Plotworld and not in the zombie world.

You can view how far away the next restart is with the command /wr (short for /whensrestart).