Avira in its free version uses different (often slower) servers and the frequency although I do not recall the exact interval is lessened, I want to say it checks for updates every 24 hours though. The answer can be found on the Avira forums though, this is a copy/paste from their forums...
5.2.1 Avira AntiVir Personal automatic update
Note
An update job has been pre-installed to update Avira AntiVir Personal every24 hours if
an internet connection is available and additionally when an internet connection is
established.
17
Avira AntiVir Personal
To create a job in AntiVir Scheduler to update Avira AntiVir Personal automatically:
In Control Center, select the Manager:: section. Scheduler.
Click on the Create new job with the wizard icon.
The dialog box Name and description of job appears.
Give the job a name and, where appropriate, a description.
Click Next.
The dialog box Type of job is displayed.
Select Update job from the list.
Click Next.
The dialog box Time of job appears.
Select a time for the update:
Immediately
Daily
Weekly
Interval
Single
Note
We recommend that you update Avira AntiVir Personal regularly and often, e.g. every24
hours.
Where appropriate, specify a date according to the selection.
Where appropriate, select additional options (availability depends on type of job):
Repeat job if the time has already expired
Past jobs are carried out that could not be carried out at the required time, for
example because the computer was switched off.
Click Next.
The dialog box Select display mode appears.
Select the display mode of the job window:
Minimize: progress bar only
Maximize: Entire job window
Hide: No job window
Click Finish.
Your newly created job appears on the start page of the Manager :: Scanner
section with the status activated (check mark).
Where appropriate, deactivate jobs which are not to be carried out.
Use the following icons to further define your jobs:
View properties of a job
Modify job
Delete job
18
Overview of AntiVir Personal
Start job
Stop job