[DahkCeles]

This is a more complicated solution that would weight recent performance more strongly and only consider the last 15 seconds.... which should shield you from awkward boss fights that have intermission phases.

• Simple average, below five seconds
• 10% moving average, beyond 15 seconds
• Hybrid between 5 and 15 seconds

local healthRecord = {}
local tickers = {}
 
local pattern = "Target will die in %d seconds"
 
local function updateEstimate(unit)
    local record = healthRecord[unit]
    tinsert(record, UnitHealth(unit)/UnitHealthMax(unit))
    local time = #record
    local dps
    if (time > 15) then
        dps = record[time-15] - record[time-14]
        for i=time-14, time-1 do
            dps = 0.9*dps + 0.1*(record[i-1]-record[i])
        end
    elseif (time > 5) then
        dps = (record[1] - record[5]) / 4
        for i=5, time-1 do
            dps = 0.8*dps + 0.2*(record[i-1]-record[i])
        end
    else
        dps = (record[1] - record[time]) / (time-1)
    end
    if (dps > 0.01) then
        local timeToDie = record[time] / dps
        print(pattern:format(timeToDie))
    elseif (dps < -0.01) then
        print("Not dying any time soon!")
    else
        print("Are you doing any damage?")
    end
end
 
local function startTracking(unit)
    if (healthRecord[unit]) then
        wipe(healthRecord[unit])
    else
        healthRecord[unit] = {}
    end
    healthRecord[unit][1] = UnitHealth(unit)/UnitHealthMax(unit)
    tickers[unit] = tickers[unit] or C_Timer.NewTicker(1, function() updateEstimate(unit) end)
end
 
local function stopTracking(unit)
    if tickers[unit] then
        tickers[unit]:Cancel()
        tickers[unit] = nil
    end
end
 
local function stopAllTracking()
    for unit, ticker in pairs(tickers) do
        ticker:Cancel()
        tickers[unit] = nil
    end
end
 
local frame = CreateFrame("Frame")
frame:RegisterUnitEvent("UNIT_TARGET", "player")
frame:RegisterUnitEvent("UNIT_HEALTH", "target")
frame:RegisterEvent("PLAYER_REGEN_ENABLED")
frame:RegisterEvent("PLAYER_REGEN_DISABLED")
frame:SetScript("OnEvent", function(__, event)
    if (event == "UNIT_TARGET" and InCombatLockdown() or event == "PLAYER_REGEN_DISABLED") then
        stopTracking("target")
        if (UnitExists("target") and not UnitIsDeadOrGhost("target")) then
            startTracking("target")
        end
    elseif (event == "UNIT_HEALTH" and UnitIsDeadOrGhost("target")) then
        stopTracking("target")
    elseif (event == "PLAYER_REGEN_ENABLED") then
        stopAllTracking()
    end
end)

PS, if you think this is a bit too complex (you wouldn't be wrong!) then consider a simple-moving-average... ie, just take the last X seconds and do a single division whenever its more CombatStartTime is beyond some threshold.