Quoted from a moderator at the MSDN forum:
Get a timestamp when you start the process

DateTime starttime = DateTime.Now;

To calculate the remaining time you will get the time spent so far

TimeSpan timespent = DateTime.Now - starttime;

From the total amount of seconds spent and the progressbar you can calculate as estimate of what remains.

int secondsremaining = (int)(timespent.TotalSeconds / progressBar1.Value * (progressBar1.Maximum - progressBar1.Value));

The time spent divided per item already processed multiplied by the items remaining will give a estimate close to reality if the amount of time per item is not fluctuating too much.

For performance and to be less sensitive for fluctuations it is probably best to update at regular intervals and not after each item has been processed.

I used progressBar1 properties to demonstrate how it connect to the progressbar, actual implementation probably should not do it like that but internal counters and limits instead.

EDIT: Fixed a type TimeStamp is not what I meant, changed to TimeSpan
It is written in C# syntax but I'm sure you can understand the concept.