yes Yes YEs and YES! lol
For one thing I doubt whether many serious people out there are building their sites live on the server if they can help it.

Personally I have a number on the go.  But for each live web site i have 3 versions of dnn on the go.  1 on my dev pc.  When i am happy then i move it to my staging server which simulates the live site including a mirror of the database etc so i can test any upgrades and break as necessary.  When i am happy then i upload to the live site.

Not everyone is in that fortunate position as they have to have someone else host for them, but give me a few more days and there will be a new hosting plan for the DNN community :)

To answer your question though, it depends where you are building your site and who is the hoster.  On saying that it should be a 10 second changeover when you are ready.  Its the getting ready bit that takes the time depending on your access to the filesystem on the live site and access to IIS.

At present your IIS should be pointing to a directory on the webserver (eg c:\inetpub\websitedirectory).  You need to create a new folder at the same level as your existing site if you can.  (e.g c:\inetpub\dnnwebsitedirectory) and install dnn, the database, the permissions for ASP.NET or Network Service etc here.  If your hoster allows it ask for a subdomain to point to the dnn site.  (so www.yourwebsite.com points to c:\inetpub\websitedirectory and ask for a subdomain test.yourwebsite.com to point to c:\inetpub\dnnwebsitedirectory).  If they do that then you will be ok as you can use it for your staging server later too.  Build your DNN site and test it as you go on the new subdomain.  When you are happy and want to go live get them to switch the path of www.yourwebsite.com to c:\inetpub\dnnwebsitedirectory.  The dnn site will be live then.  if they wont let you have a subdomain see if you can get a virtual directory pointing at this dnn path (you will then have to access it from www.yourwebsite.com/virtualdirectoryname).  It all depends on your hosting situation.

If you tell me where you are building your site (and i would recommend you use your own pc to start with) and who hosts your current site for you (or if you are changing hosters too) then i can give you specifics on how to do it.

It really is simple to do and no one will notice any downtime (though if someone is in the middle of using your site or on a session they will notice the change and lose any session information too - try and plan for a quiet time).

Hope it helps
Mark