According to Help:Using_a_container it is 15 minutes

Field container

A field container is a useful storage device for Agents to exchange items. Easy to use: buy one on the market, place it inside your activated robot's cargo hold before deploying onto the terrain. While you're outside the terminal, open your cargo, right-click on the field container then place it on the ground. By giving the container's security code, you can prevent any unwanted Agent from opening it.

Anyone who knows this code can put in or withdraw items. When a container is deployed it cannot be put back into the cargo hold. If no one opens the container for 15 minutes, it destroys itself.