The need of full backups is more obvious when more time goes by. When recovering from the incremental backups you go through all the changes after last full backup in order to recover the current state of every file.
How often you should take full backups depends on how long you need to keep incremental data i.e. how distant past versions you may need to recover. You can make calculations on how much data your backup will have based on variables:
- size of the one full backup
- average changes per daily incremental backup
- days between full backups
- total days you keep backups
- do you need to store incremental data only after last full backup or all of them
For example if you have 700 GB of total data with average 3 GB of daily changes and you keep backups for half a year, the total amount of data in your backups:
- Weekly full backup: 700 GB * 26 + 3 GB * 6 = 18 TB of data to backup
- Monthly full backups: 700 GB * 6 + 3 GB * 30 = 4,3 TB of data to backup
- Quarterly full backups: 700 GB * 2 + 3 GB * 90 = 1,7 TB of data to backup
And if you only take one backup and then keep taking incremental backups for 4 years, you will end up with original 700 GB full backup and 3 GB * 365 * 4 = 4,3 TB of incremental backups you need to go through, when you perform a full recovery.
There isn't one correct solution for time between full backups, but with these variables you can count what you value the most in your situation.