A 6 to 8 pound Cornish hen? You must be thinking of a roaster. A cornish hen is only averages maybe 1.5 pounds each. They should only take about 30 minutes on a rotisserie.
That depends on how many pounds I have a rotisserie built in my grill, when I do a 6 to 8 pounder, it usually takes 3 hours on low and the taste of a rotisserie chicken is delicious but I'm sure you already know that.