Most backup schedules involve periodic full backups interspersed with incremental backups of the transaction log. There is no simple rule for deciding how often to make backups of your data. The frequency with which you make backups depends on the importance of your data, how often it is updated or changes, and other factors.
Most backup strategies involve occasional full backups, interspersed by several incremental backups. A common starting point for backups is to perform a weekly full backup, with daily incremental backups of the transaction log. Both full and incremental backups can be performed online (while the database is running) or offline, on the server side or the client side. Archive backups are always full backups.
The kinds of failure against which a backup schedule protects you depends not only on how often you make backups, but also on how you operate your database server. See Configuring your database for data protection.
You should always keep more than one full backup. If you make a backup on top of a previous backup, a media failure in the middle of the backup leaves you with no backup at all. You should also keep some of your full backups offsite to protect against fire, flood, earthquake, theft, or vandalism.
You can use the event scheduling features of SQL Anywhere to perform online backups automatically at scheduled times. See Automating Tasks Using Schedules and Events.