One solution would be to have two download buttons.  One would connect to a fast connection for DSL and cable, and have a 15 minute limit.  Just about any file on sharecg could be downloaded on DSL or cable in 5 minutes.  The other button would be for modems, and have a 4 hour limit, but have a maximum transfer speed of 56k.  It is impossible to transfer as much through a 56k modem in 4 hours as you can transfer on DSL or cable in 45 minutes. 

Another option is to allow the visitor to select to downlod the file in 1, 4 or 10 megabyte chunks, then have the server slice the file into the chuncks.  Creating a program to slice and reassemble files is very easy.  I wrote my own over a decade ago, and am willing to create a Windows version for free.

                                 -David