[SheaColandri]

My brief research led me to the following info:

"Real-world individual white phosphorus (WP) smoke rounds burn for 1 to 1 1/2 minutes --- other rounds burn 3 to 4 minutes. Duration of artillery smoke after active burning stops depends on number of rounds fired, their distribution, plus environmental conditions such as humidity, temperature, and wind."

Also, the thermal imaging obscuration of WP smoke fades within seconds.