Why does my backup take so long and never completes?

Thanks for posting in the Apple Support Communities Hannasjim,

The amount of time an iCloud backup takes to complete can vary depending on a numbers of factors, including whether it's your first backup, your internet speed, traffic on your network, and the amount of data being backed up.

If your iCloud backup is taking longer than expected to complete

1. In iOS or iPadOS, go to Settings > Wi-Fi. Check that you're connected to Wi-Fi. 

2. Check your upload speed.

3. Leave your device connected to power and Wi-Fi for 24 hours. If your backup still isn't done, archive important data then contact Apple Support. 

If you can't back up to iCloud

It's also important to make sure the amount of available storage in iCloud, is large enough to accommodate the amount of data that needs to be saved from your iPhone. You can find useful information to help, here: What's the difference between device storage and iCloud storage?

Cheers!