Please note there is no time limit option either on Social Miner or UCCX side that will decide how long an e-mail will stay in the Email queue. As long as agents are available to take them, UCCX will keep on routing. The moment there is no one then the email will still remain in the queue unless someone delete them directly from the Exchange server or set a retention policy there.
In nutshell, it is more of a Exchange thing rather than a Social Miner or UCCX setting.
Regards
Deepak