Player synchronization

I have a question :

during update cicle, what amount of time is better to interpose between sending of sync message for players ?

I tryed with a second but it makes the queue reaching max size after a wile.

Is there a way to empty the queue if a message is not delivered by a second ?

Well, it depends on the priority of the object how frequently to send updates.  For a FPS I might send an update every 5ms but for an object that is far away from the client (but still in the viewing area) the server may only need to send information to that client every second.