I'm not aware of a way in which the stock game shows how much longer a poison cloud will hang around, which makes it difficult to make decisions about it. Perhaps this is something a mod could add?

Thank you for raising this issue.

I took a look at the code and it appears that the TileEffect expiration times are defined in a GetLayerExpireTime method inside TileEffectState.cs When a tileEffect is created/applied, a turnCreated parameter is included. This turnCreated parameter is used in a HasExpired function in combination with the to calculate whether to remove the effect.

Finding out how long any particular tileEffect has remaining doesn't look too difficult. Working out how to display a turn-left counter on each tile will be more of a challenge, and the display would only work for the host of the game.

Whether we're able to add a countdown timer or not, your idea has inspired me to create a new rule for controlling the durations of the tileEffects... It's totally the opposite of what you asked for, but I like the idea of poison gas lasting longer and spreading further, so i'm going to have a play around with that :-)