dnd 5th – How long does the line of sight that you can create as an action using the flame inlay spell?

I've always interpreted the Investiture of Flame spell as having a flamethrower effect, similar to Aganazzar Scorcher, but one of my players proposed a different interpretation.

Here is the relevant segment of the spell:

You can use your action to create a fire line 15 feet long by 5 feet wide in the direction you prefer. Each creature in the line must make a save dexterity roll. A creature takes 4d8 Fire damage in case of failure of a save or half less damage if successful.

My player pointed out that the spell as a whole corresponds to a concentration of up to 10 minutes, while no duration has been specified for the line of sight. This led them to believe that the lines of fire remained frozen.

I think it's just a bad wording on the part of the authors, which supposed that the reader would interpret it as an instantaneous effect, rather than a dangerous spell, because such spells almost always have the "deal" clause. not wounds when a creature starts his turn there or enters it for the first time. "Nevertheless, I wanted to check a second time to be sure not to overpower this 6th level spell.

How long does the fire line last? Is it instantaneous or does it stay there until the end of the spell?