I got some questions about this issue.
a. If using incremental backup, Do we need to a full backup once a week?
b. How could i change the time of backup?
c. How could i know: What time does the backup start? And, how long does it last?
We can not judge how often you need backups updated, only you can decide that.
To change your backup time, edit your crontab time with this command in ssh
Look in your crontab to see the start time of your backup. The time it takes depends on the backup type, processor speed, load at the time of backup and of course the amount of files to back up. In other words, this is impossible to guess.