That option will make the client download enough units to keep itself busy for the time period you specify. It isn't 100% exact (around 99%) but its the best you can do for a specific time period.
It should only flush by itself when it needs new packets or has a reason to contact a keyserver.
However to stop it flushing until its buffers really are empty you can select an option in the same menu called 10) Additional buffer-level checking, and setting this to option 4. That tells the client to work on a project until its buffers are empty.
Also, if you really want complete connection control you can use option  5) Disable buffer updates from/to a keyserver, and set that to yes. This will mean that the client will not auto-connect to a keyserver and will only update its buffers when you tell it to (bad thing is, if you dont it will crack random blocks). If you do select no, the option to change your server/connection settings disappears but, dont worry, it still works the same as you left them.