I don't believe it's health-based. Take a look at a few random logs and you'll pick up a pattern, regardless of the raid's DPS.
He's got five timers for Mutated Infection, 14, 12, 10, 8 and 6 seconds. The raid will get four applications at each timer, then 6-second intervals until dead. (I have noticed that there seems to be a delay in applications 4 and 8, however.)
Links to triggers on Wowhead:
14-second trigger
12-second trigger
10-second trigger
8-second trigger
6-second trigger
Also note the description in this
blue post: "The previous iteration of Rotface in 10 player difficulty used (and still uses) the same timer as the 25 player difficulty version when dispersing its Mutated Infection ability."
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