At a guess (taking into account you haven't done it before)....
Half an hour to download Ubuntu/Mint etc.
An hour (max) to create a LiveUSB installer (as it's your first time)
An hour to install (max)
An hour (max) to follow the instructions on this forum to fix a few small issues you may have, like getting the card reader, mic socket, and wireless to work.
10 mins of automatic updates.
Couple of hours of learning how to use the package manager(s) and customising the install with any
extra software such as Skype, Veetle, etc.
The reason I underlined
extra is because most of the software you will require will have been installed along with the OS .. such as Media players, Office suite (LibreOffice), PDF reader, Photo viewer/organiser, Archive manager, Web browser (Firefox 7), Email client (Thunderbird 7), Torrent client, IM client, etc.
Done

The "Update Manager" will then automatically keep all of your software up-to-date .. ie. when a new version of Firefox (or any other software you have installed) is released, it will be automagically updated for you.
Probably best to set half a day aside, if you want to do it all in one go .. but you can obviously do the customising at your leisure.
Lets put things this way .. I could probably do it from scratch in about 2 hours .. that includes downloading, creating the LiveUSB, installing, updating, and customising .. but you may need to add some reading time.
If you know how, its MUCH quicker than a Windows install .. which takes about the same time for the OS install, but then hours of updates (did a Vista install the other day, the updates alone took 13 hours to get it to service pack2 .. I'm NOT kidding), then you have to install all your applications and drivers yourself, not to mention AV

If anything I've over estimated the time it should take .. I can point you to all the relevant instructions if you wish ? .. or pull it all together in one place ?