ilovesimio

I'm trying to model a system and I was wondering if there was a way to model the people who leave the system due to excessive waiting times. There were 2 logics I came up with but do not know how to model: 1. When they enter the system, there are already 50 people in queue so they decide to leave the system. 2. They enter the system and wait in the queue. After 30 minutes pass of waiting in the queue, they decide to leave the system. Any help would be greatly appreciated! Thanks!