About backing up the transaction log
By default a transaction log is truncated after it has been backed up. However, it is not truncated following a full database or differential backup. Databases must be set in either full or bulk-logged mode.
The main factors in deciding how frequently to back up a transaction log would be the following:
Conservation of log space.
How close to the failure point you must be able to recover in case of a disk crash.
During peak periods in a high transaction environment, it may not be unusual to back up the transaction log every few minutes. (Note that legacy SQL Server policies can only be scheduled to run in hourly increments.)