Mortar smoke was an effective way for a battlefield commander to impair the enemy's view. In fact, mortars were the fastest way to get smoke on the battlefield as artillery fire missions usually took longer to coordinate than setting up and firing an infantry mortar.
Interesting mechanic to allow use one time per mortar unit. Another would be to treat it like infantry smoke - on a successful smoke roll, you can place a smoke counter, otherwise OPS COMPLETE. Add some risk to the call for mortar smoke - just like its infantry counterpart.
As for length, mortar smoke rounds burned for about a minute, so that's probably on par with infantry smoke grenades - the same smoke mechanic could be used.