It seems that there is no limit to how much work gets generated; only a certain number of robots get dispatched each tick; if you queue thousands of orders you get to the point that it takes a semi-infinite amount of time for manually issued orders to have robots assigned. Ideally there should be no more than 15 ticks worth of automatically generated work in the queue at any one point in time (humans consider a delay under 1/4 second as instant, thus 15 ticks).
