total = 100	;Number of loop iterations.

Gui, Add, Text,, ETA:
Gui, Add, Text, vETA ym, 0:00:00
Gui, Add, Progress, xm w500 h50 cGreen vProgress Range1-%total%
Gui, Show

StartTime := A_TickCount
Loop, %total%
{
 Sleep, 200	;Replace the sleep with commands to execute.
 GuiControl,,Progress, %A_Index%
 time := A_TickCount - StartTime
 SumX += A_Index
 SumY += time
 SumXY += A_Index * time
 SumX2 += A_Index**2
 m := (A_Index * SumXY - SumX * SumY) / (A_Index * SumX2 - SumX**2)
 b := (SumY - m * SumX) / A_Index
 ETA := (m * total + b) - (m * A_Index + b)
 T := A_Year
 T += ETA/1000, Seconds
 FormatTime, ETA, %T%, H:mm:ss
 GuiControl,,ETA, %ETA%
}

GuiClose:
ExitApp