Ok
Ima try to give you an answer.
May be wrong but I'll try.
13 Gigabytes = 109,051,904 Kilobits.
86,400 seconds in a day.
Your connection allows for 256 Kilo bits per second of upload.
So 86,400 seconds * 256Kbps = 22,118,400
Then
109,051,904/22,118,400 = 4.9304 Days ?