- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks how many seconds it should take the unit specified to reach the health percentage you set, compared with the number of seconds you set. The estimate comes from recent damage, healing or both, as Mode says.
Why you might use it
Estimates how many seconds until a unit reaches a health percentage, from the recent rate of damage and healing on it. Use it to prepare for an execute phase, such as pooling rage for Execute when the target will reach 20% within a few seconds, or to hold a cooldown for a boss phase that starts at a set percentage. Mode picks whether damage, heals or both feed the estimate. It does not exist on Retail or WoW Forever, where health and the combat log are hidden from addons. See also Time Until Health and Health Percentage.
Example
time until your target's health reaches 20% is less than 5 seconds with Damage Only
Settings
What each setting in the condition editor means, from the game's own help.
Unit - The unit this condition uses as the target of comparison.
Health - The target health the rule is waiting for Unit to get to as a percentage of their total health.
Operator - How to compare Time Until Health Percentage with the Seconds value. This comparison uses basic mathematical comparison operations.
Seconds - How many seconds (estimated) until Unit reaches the target health percentage.
Mode - How the burn rate should be calculated, which is used to estimate the time to target health.
Damage and Heals - Both damage and heals on Unit will be factored into the 'burn rate' (essentially mitigating damage with heals for a realistic burn rate)
Damage Only - Only damage on Unit will be factored into the 'burn rate', which will cause some spikes in time-to-target if they are healed.
Heals Only - Only heals on Unit will be factored into the 'burn rate', essentially pretending they are not being hit at all.