how do I know the time involved in updating the firmware of my samsung I5801 phone via KIES The official update for samsung I5801 is available now. 

Has anyone done it before (using KIES, any samsung phone)? 
How long it will take to update the firmware via Samsung Kies pc suite? 
What are the additional things to do before and after I update my firmware? 
(* I have taken backup of my contacts, messages and applications on the phone's memory card&nbsp;* I will reset my phone before and after updating the firmware using the settings -> factory reset option)

If something goes wrong then what can be done to revert.

Please help me as I don't want to fall in trouble after attempting an update.

Thanks
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.