Check out the time documentation for lua. What you can do is set an idle delay variable, grab the time the mob gets to the navpoint, start a loop, then set it's mode to "idle", and check the "end time" variable (start + delay) against the "current time". If current time (set in the loop) is greater than or equal to "end time" (set before the loop), set your mob's mode appropriately to "roam", else continue the loop.