Transferece Speed

Well all you need to do is

Time(seconds) =    Filesize (bits)
                           -----------------------------
                            User's connection speed (bits per second)

You could just do it for popular connections speed like 56k etc. because it is quite hard to get the users actual connection speed.

I.e. If you are downloading a 1MB file on a 56k connection your script would have to do the following

8388608 (1MB)
--------
56000    (56k)

and would get 149.7 seconds