Oil Flask: How long does the bonus damage last?

[banthafett]

As an action, you can splash the oil in this flask onto a creature within 5 feet of you or throw it up to 20 feet, shattering it on impact. Make a ranged attack against a target creature or object, treating the oil as an improvised weapon. On a hit, the target is covered in oil. If the target takes any fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil.



Does someone who has been doused with oil via an oil flask take five extra damage every time they take fire damage for the full one minute duration of the flask, or are they no longer considered doused once they have taken their first instance of fire damage?

[Zanthy1]

I've ruled it that the dousing lasts the regular duration, however once that is lit I roll 1d4 for the amount of rounds they take fire damage (though I rule it as 1d6 each round they're on fire).