Calculate Time Remaining

I'm surprised no one has answered this question with code!

The simple way to calculate time, as answered by @JoshBerke, can be coded as follows:

DateTime startTime = DateTime.Now;
for (int index = 0, count = lines.Count; index < count; index++) {
    // Do the processing
    ...

    // Calculate the time remaining:
    TimeSpan timeRemaining = TimeSpan.FromTicks(DateTime.Now.Subtract(startTime).Ticks * (count - (index+1)) / (index+1));

    // Display the progress to the user
    ...
}

This simple example works great for simple progress calculation.
However, for a more complicated task, there are many ways this calculation could be improved!

For example, when you're downloading a large file, the download speed could easily fluctuate. To calculate the most accurate "ETA", a good algorithm would be to only consider the past 10 seconds of progress. Check out ETACalculator.cs for an implementation of this algorithm!

ETACalculator.cs is from Progression -- an open source library that I wrote. It defines a very easy-to-use structure for all kinds of "progress calculation". It makes it easy to have nested steps that report different types of progress. If you're concerned about Perceived Performance (as @JoshBerke suggested), it will help you immensely.

Why not?

(linesProcessed / TimeTaken) (timetaken / linesProcessed) * LinesLeft = TimeLeft

TimeLeft will then be expressed in whatever unit of time timeTaken is.

Edit:

Thanks for the comment you're right this should be:

(TimeTaken / linesProcessed) * linesLeft = timeLeft

so we have

(10 / 100) * 200 = 20 Seconds now 10 seconds go past
(20 / 100) * 200 = 40 Seconds left now 10 more seconds and we process 100 more lines
(30 / 200) * 100 = 15 Seconds and now we all see why the copy file dialog jumps from 3 hours to 30 minutes :-)

Make sure to manage perceived performance.

Although all the progress bars took exactly the same amount of time in the test, two characteristics made users think the process was faster, even if it wasn't:

1. progress bars that moved smoothly towards completion
2. progress bars that sped up towards the end