Reprint from

Edit ~/Library/LaunchAgents/com.dropbox.DropboxMacUpdate.agent.plist so that StartInterval becomes 2592000 (a month). Interestingly, mine is 3623, instead of 3643 as the author says.

Mysteriously, this file will be overwritten somehow, removing the write access via chmod -w does not persist either. In the end, I had to resort to marking the updater non-executable, restore it right before I want to update myself.

cd /Users/albert/Library/Dropbox/; chmod +x *; ./DropboxMacUpdate -check periodic; chmod -x *