"... running at around 1700 rpm come to a dead stop ..."
You left out an operative in between "rpm" and "come". Should? would? could? will? must? might? can? wants to? better not or I'll tell your father when he gets home?
In general though, the coast-to-stop time is dependant upon the load, not the motor. High inertia combined with low friction would mean a long coast time, low inertia with high friction would mean a very short coast time. Any combination variables of those two load components will result in an infinately variable outcome. In other words, no way to tell with the information you provided.
That said, conveyors are typically low inertia high friction, and combined with a 30:1 gear reduction, it is likely that it will stop quickly. But is the conveyor load going down hill? Is there a flywheel or dancer on it that has enough mass to keep it going? Is the gear reduction going the other way (I can't imagine a conveyor pulley running at 51,000rpm, but hey, who knows)?
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
