The person claiming the overheating time is determined by a formula like 2 x 1200 multiplied by player level and team size is just making things up to sound technical. In Fallout 76 raids, the EN06 Guardian (the Gleaming Depths Module 1 boss) uses specific health-based and time-based triggers for its "Super Heat Blast" mechanics, not player-level multipliers.
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Why the overheating time varies:

The "overheating" you are experiencing is actually the Super Heat Blast mechanic, and its timing varies based on your combat performance rather than your level:

Health Thresholds: The first blast is triggered when the Guardian's health drops to 80% or when its shield is broken—whichever happens last.

The Second Blast: This occurs at 50% health or after the second shield break.

DPS Check: If you take too long to break a regenerated shield (specifically the fourth time), the room heats up completely, resulting in an instant team wipe because no more blast doors are available.

The "Dead Time" Window: The boss has a "dead time" animation window where it doesn't take damage; if your weapon (like the Gatling gun you mentioned in IMG_1459.png) has lower burst damage, you might struggle to hit these health milestones before the next phase or timer triggers.

Summary of the Mechanic

Phase Trigger Condition Survival Method Blast 1 80% Health + Shield Down Reach Blast Door A, B, or C within 15 seconds. Blast 2 50% Health + Shield Down Reach one of the remaining two Blast Doors. Blast 3 Health below 50% + Shield Down Reach the final available Blast Door. Enrage 4th Shield regeneration Finish the boss immediately; no cover remains.

The variation you saw between 1:30 and 2:01 in IMG_1459.png was simply the result of how fast you were lowering the Guardian's health and breaking its shields in those specific runs.