Reference: Watch Items
Watch Items are set to run at specified intervals, as defined in the Watch Item Properties window. Watch Items will run, then "pause" for the length of the interval minus the length of time the last interval took, minus a small amount of overhead. This is to attempt to run the Watch Item for the specified interval. For example, a Watch Item is scheduled to run every 5 minutes, the Watch Item's last response time was 552ms and the overhead was 22ms.
30000 (interval) -552 (last response time) -22 overhead ----- 29426
After the above run, the watch item will "pause" for 29426. Generally, the margin of error is +/- 2 seconds.