You should only need to sync after the Kindle is restarted. If the ads really annoy you (but the nag to "connect to download the latest ads" doesn't) you can always use the
KUAL Firewall with BBB filter to firewall Amazon out of your Kindle. That will block ads, the Kindle Store, syncing for Amazon's timeservers, everything else, and so forth.
Then use
KUAL DateTime Button to update the time.
Only necessary after the Kindle is restarted.