Ward disappearance time

When someone places a ward, does it disappear from enemy at random or there are conditions that affect when the enemy can't see the ward anymore? I mean like I want to hit a ward and sometimes I can hit it for a long time, but then sometimes it disappears after a second.

We're testing a new feature that gives the option to view discussion comments in chronological order. Some testers have pointed out situations in which they feel a linear view could be helpful, so we'd like see how you guys make use of it.

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