how do I know the time involved in updating the firmware of my samsung I5801 phone via KIES
The actual update should take at very most half an hour, the download time may vary though.  Other than a backup, I don't see what else you'd need to do.

If you have trouble, come back and post a separate question with details.  The update usually either fails entirely (in which case you can try again) or works fine, you shouldn't end up in some half-updated broken state.