Currently, we get barely any useful information from the burning icon on the enemy. If you see the burning indicator at 50 %, it could be 50 % of a 10 second burn (which means the burning lasts for 5 more seconds), or it could be a full stack (30 seconds) in which case you should know that it’s pointless to stack it any more.
My proposal is to have the burning icon always at 30 seconds max (or 60, I’m honestly not sure what’s the max stacked duration atm). That way if you see the burning icon at 25 %, you know that the burning condition will last for precisely 7.5 seconds (or 15) every single time.