I find it best for most fitness levels when I do 45 seconds for each exercise and a 15 second recovery where they go to the next exercise. During that 15 second recovery I assist any participants that may need a modification or not sure what they are suppose to be doing. I typically have 10 circuits with various exercises consisting of upper body, lower body, cardio, and core. I place signs up at each station so the participants know what they are to do at each station which saves me from running around instructing each participant.
A second way that I have successfully ran a circuit too is all of the above, but have the participants partner up. Group 1 does the strength exercise at the station while Group 2 does the cardio portion then they switch before moving on to the next station. This works really well for bigger classes or if more people show up than you expect.