How long should it take to backup an Iphone to Icloud

How large is the backup

Remember, you're moving files, pictures, documents etc over the thin air to the internet. The backup will depend on these things: how large the backup is - moving a few hundred megabytes is a lot faster than moving a few gigabytes; how solid your wifi connection is - you usually have a much better connection right near the router than you would upstairs at the other end of the house; how fast your internet connection is - you'll move data much faster on a gigabit connection than on a dialup connection; finally, on what else the phone is doing while you're doing the backup - it will back up a lot faster if it's sitting still next to the router than if you're talking on it, playing games on it, surfing the net on it and so on.

How long is a piece of string?

Impossible to give an accurate answer with no details.

The whole backup to iCloud process is dependent on many variables. Most out of your control.

The very first one will be the largest/longest. iCloud does what are called incremental backups. Only changed data is added after the first backup is completed.