Are you bothered about the alerts during the weekend as well as the weekdays?

If so you will need to set the alert to 62 hours for a backup threshold if not then 14 hours.

To do the change, open up the log shipping configuration from the primary server and database and press the backup settings button, on that screen you should see "Alert if no backup occurs within" and you can then specify the threshold.

Edit.

You will then have to look at the restore thresholds as they will then start to error if it cannot restore due to no backups being taken.

Same as above, but then select the secondary and click the elipsis button next to the secondary database, and on the restore transaction log tab, change the "Alert in no restore occures within" threshold to match.