How often is the time synchronized - is there a schedule?
The NTP protocol syncs constantly. When the ntp server start it will send a burst of sync packets pretty quickly, and then when it stays in sync it will back off to typically sending a packet every ~1000 seconds.
Is there a log that says when the time was synchronized?
NTP logs extensively. It tells you offsets and so on. You can see the current status by running the command
Is it possible to control when the time should be synchronized
With ntpdate alone you could set a cron task to only sync periodically, but you really shouldn't do that. Instead use the full ntp server, and let the server synchronize things. It will dynamically pick the best values for your system to keep you to a very high level of accuracy.
I strongly suggest that you take some time and read how the NTP protocol works.