You need to know what the rules or what section of the rulebook the class is being judged under. I don't know where Dobbin showed, but I've never heard of ride times being posted for jumpers, just for combined traning/combined tests.. In schooling classes or jumper divisions run within hunter shows; there's a posted order of go, just like for an eq class, but no accurate way to determine your ride time other than hurry up and wait and count how many trips ahead of you in the order.
Most jumper classes require you enter the ring, face and salute the judge. After the judge returns the salute, a signal will be given, usually a whistle or buzzer, and you have a set period of time, usually 60 seconds, to cross the beginning timers and start the course. If you make too large an opening circle or approach and cross the timer line you can be disqualifed, so make sure you understand where the timers are.