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It depends greatly on what the "something" is. If you can assume that the amount of time to process each line is similar, you can do a simple calculation:
TimePerLine = Elapsed / LinesProcessed
TotalTime = TimePerLine * TotalLines
TimeRemaining = TotalTime - LinesRemaining * TimePerLine
If you know the percentage completed, and you can simply assume that the time scales linearly, something like
timeLeft = timeSoFar * (1/Percentage)
might work.