You can retain VM backup data indefinitely if retention is disabled, meaning all restore points will be kept. However, if you enable retention policies, you have two options:
Basic Retention:
Version-Based Retention: You can specify to retain up to 999 versions of backup data.
Day-Based Retention: You can choose to keep backups for a specified number of days, also up to 999 days.
GFS (Grandfather-Father-Son) Retention: This allows you to maintain a hierarchical backup structure, including daily, weekly, monthly, and yearly backups.