I've found the timetables of COTRAL bus company at its website. However, I find them difficult to interpret.
From Monday to Saturday, according to these timetables, there are three buses going from Viterbo (Piazza Giordano Bruno) to Orvieto (Piazza Cahen). However, how long does this bus take?
The information called "percorso" is also difficult to interpret. Are these the bus stops from the beginning to the end? Are there additional bus stops not listed there?