The time interval of the update is set at 7 days by default, this can be changed in the registry:
Start the Registry Editor
Go to HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Services \ W32Time \ TimeProviders \ NtpClient \
Double-click the SpecialPollInterval value, and change the Base of the Value data to Decimal
Now change the Value data to the time interval you desire, noting that the time is given in seconds (so for 1 day the value would be 86400, while the default (7 days) is 604800