To figure out how long it takes to download a file, you take the size of the file and divide it by the rate (keeping the time unit).
So for example if you need to transfer 50MBytes and your transfer rate is 1Mbyte a second:
50M/1M ( A Second ) = 50 (Seconds).
50M/2M ( A Second ) = 25 (Seconds).
You can also just type "50 Megabytes at 1 Megabyte a second" into wolframalpha: http://www.wolframalpha.com/input/?i=50+Megabtyes+at+1+Megabyte+a+second
How Fast am I?
I use the speedtest.net, a web testing tool that shows your network download and upload speed.
If it Takes Too Long:
If it is going to take a really long time for a large amount of data. You can use "sneakernet" (Transfer the file on storage over mail or on foot). For instance, Amazon offers this service with their import/export service.
Get your Units Right:
File sizes are usually in Bytes, where as network speeds are in bits. In general there are 8 bits per byte. If you just cheat using wolfram alpha this makes it easy, but keep in mind most speed tests are in bits.