Backups should be performed daily. For additional security, a grandfather-father-son scheme (multiple backups taken at different intervals) should be applied.
In an SME with a five-day work week, a full set of backups taken under this scheme could look like this:
four son backups (daily from Monday to Thursday; Friday is kept as a father backup)
four father backups (weekly, every Friday of the month)
twelve grandfather backups (monthly, on the last day of every month)
Using relatively little storage space, this scheme ensures that all data are stored in the short, medium and long term and that any data lost can be restored from a previous level, at least to a large degree.