how long do it take to backup

Posssibly.

The backup speed can depend on the amount of data being backed up and the speed and reliability of your Internet connection.

How to back up your iPhone, iPad and iPod touch – Apple Support

The link posted earlier talks about different ways to do backups but doesn't actually discuss what affects backup time which seems to be the focus of your question.

From your image your WiFi connection is not at full strength which suggests to me that is what is causing things to take a long time. From my experience once you are down to 2 bars you definitely start onserving decreased transmission rates. As stated in a previous post, transfer time will also depend upon how much data you are trying to transfer.

Backing up to a computer would almost certainly be faster but your decision as to which backup method to use will depend upon where you want your backups stored.