Regarding the plat only two rules are important:
- hero hp is less than 80 %
- hero suffers no damage
With the help of my alliance mate notoriouscrozz I found an estimate by having a mech 1v1 against gale.
Shooting her below 80 % and stopping to fire resulted in about a 1.5 sec delay to activity.
Shooting her again removed buff, only to reapply after the exact same time over and over.
I am not 100 % sure but the reapplication time is between 1-2 secs based on the 1v1, also observed by teammate.
Regarding the mark buff:
The damage buff is a positive buff on your team shown by the blue variant of the skill, and the stealth denial/mini taunt is a negative enemy debuff shown by the red variant. Can be purged by Heimlock and Phoenix.