At the NCT races, (160-230entries) typical practice day is ran open from 8-12, then 1/2 hour blocks of classes, electric, gas truck, monster truck, 1/8b, 1/8t, and then repeat 2 more times, and then the last hour or so is usually open. And typically there are enough people with PT, of course this is a ROAR event, so everyone ought to have a PT, that computer bugs can be worked out in each class by simulating races during the 1/2 hour blocks.
HTH
Rod