Backup speed depends on your internet connection speed, and 1GB takes 2 hours on a 1Mbps connection. When I had ADSL my speed was 4Mbps. Iphone 6 came in 3 sizes, with 128GB the biggest (and 16GB the smallest). So it could take 60 hours on an ADSL line My current connection is 46Mbps. You can determine your speed using speedtest https://www.speedtest.net/

The iphone backup is differential, so will be fast once you do the first one (typically minutes as only new data has to be backed up).