512kbits per second = 64000 bytes per second * 60 = 3.8MB per minute.
256MB / 3.8MB = 66.66 minutes.
Realtity will be much different. You will not maintain 512K it will depend on the download site, how busy your ISP is etc.
105 minutes to download 256MB means you avaraged 2.44MB a minute @ 406K