In the system i know, the authentication is performed at IMSI attach, Loc Upd and every call setup (OC or TC). There is no problem of overload or too long call setup time : the duration of the authentication is few hundreds of ms.
You must ensure that there is no overload on the AUC database. Beside there is a parameter which controls the number of authentication that can occur simultaneously. This is to prevent the equipment from break down. If it reaches this threshold all the excess authentication will be discarded.