It depends on what product you use.  Please note that even if the barrier is placed outside in the sun, the contents of the moat is shaded.  It will never get direct sunlight.

If you use water, replenish when it evaporates.  Depending on the conditions, it could last up to  60 days.

If you use dishwashing liquid, you can refresh the soap with some water every 20  to 30 days without having to put new dishwashing liquid in.

If using Cooking Oil, it will last all year.

If using Biodegradeable Lubricants.....................for a long time!
Q:   How often do you have to replenish the household product in the moat so that the barrier remains effective?