@Huracan not sure about Free, I am using the paid version atm, and the default setting is to check for updates every 2 hours.
Not saying there *are* updates every two hours, but at least it checks for them =) I never checked until you asked but the log files show it looks for updates every 2 hours without fail unattended. Some of the logged updates are manual updates just because I am anal retentive (note the candy cane). For Xmas day it appears all the updates at 36 past the hour were automated. I turn the machine off at night while I sleep so you'll not see any updates for a few hours each night.
Maybe someone running Avira Free could check what the default update schedule is set at as shown in the second window. Looks like Pro will accept checking as much as every 15 minutes. Not sure about Free.
EDIT: Checked the individual logs this morning and there was only the 1 virus defs update all day yesterday even though it checked every 2 hours.
This page shows all the virus definition updates over time if you want to see how often they update compared to competitors:
VDF History