Depends on what type of installation it is. Installing EPiServer usually means installing EPiServer Deployment Center along with the CMS which really just involves running setup.exe and clicking next a number of times.
With Deployment Center installed you won't have any web sites but a tool for creating web sites.
Installation of that on a local machine or server which you have remote desktop access to, which has SQL Server and .NET 4 installed already: 5-10 minutes if you've done it before. A little more if you haven't and need to spend some time looking for the correct download :)
Setting up a site and configuring it is something different and much harder to estimate as it depends on how the site has been built (if it has), how many servers are involved etc.
So, in general I'd say there are two valid responses to your question:
1) Not more than a 5-30 minutes.
2) Define "install" :-)
Installing EPi by itself is np. Only a few minutes of work there.
Installing EPi + moving a complete site to this server can usually range between 12h to 60h depending on the number of external dependecies the site has. Typical small problems that needs to be solved along the way: database connection, IIS settings, security settings for the files, creating deployment script to automate future deployments (recommended), DNS settings, installing .NET framework, setting up logging, VPP files, performance tuning like switching on gzip etc, fixing a decent file structure on the server and testing of site.
The big part will usually be to set up external connections and firewall problems though.
Hey, I am going to estimate how long it will take to do the installation of a Episerver CMS 7, and I dont have a clue. Does anyone have any idea?