The free storage for transferring to a new device expires after 21 days (you can get an additional 21 days to restore to your new device if necessary or contact Apple Support for more). See the following from Get temporary iCloud storage when you buy a new iPhone or iPad - Apple Support
How much time you have to restore your temporary backup
You have 21 days to restore your temporary backup to your new iPhone or iPad before your temporary iCloud storage expires and your backup is permanently deleted. Do not erase your current iPhone or iPad until you restore your temporary backup to your new device.
If your new iPhone or iPad doesn’t arrive within 21 days, you can get an additional 21 days to restore your temporary backup before it’s deleted. On your current iPhone or iPad, open Settings and tap Keep My Backup Longer.
To request additional time to restore your temporary backup before it's deleted, contact Apple Support.
Beyond that, the option is to pay for a storage upgrade. See the following from that same Apple Support article:
If you want to keep using iCloud to back up your device, you can upgrade to iCloud+.
Also, you can free up some of your iCloud storage by deleting the backup of the old device (after you no longer need it to restore to a new device) by going to Settings > [username] > iCloud > Storage > Backups > [old device name] > Delete Backup.