[zzo38computer]

My suggestion is: Display a plain number (rather than a percentage); but only display the percentage if the total required is actually known too. For example, if you need to download a two gigabyte file, and the file size is already known, then it can calculate the percentage, but if the file size is not known, then it should only display the total number of bytes downloaded.

This is separate from time estimations, though. The progress may not run at a uniform speed, which is OK; just mention the percentage of progress if known, or a plain number if not. If there are multiple steps, display the progress for each individual step, and document what the steps are.

In a few cases (e.g. hashcash), the progress is not known, so just a "spinning" indicator (or displaying a plain (non-percentage) number which just increases while it is executing) might be used instead.