Harris has hit on the same points I consider. Another point is my goal. I always have a goal I want participants to achieve for each workout. For example, to build strength, to improve flexibility and ROM, to work in 75-85% HR max, etc. Each session has a goal and depending on that goal can also depend on how I structure my sessions and rest time.
Have fun with it. Be flexible and able to change the intervals quickly if you see its not working for the majority of the group. Also, consider having the group broken into teams if you have people of different abilities. When working with a group, I find its best to come with a plan, and to remain flexible enough to change that plan if you find it isn’t working.
It depend on a number of factors. Here are 3 factors I keep in mind when I design my circuits:
-Age of the participants
-Level of intensity I want to establish for the class
-Participant’s fitness goals (for example there is a difference between a tactical operation group vs. a group of clients who just want to get in shape and lose a few pounds)
In my opinion and without knowing any of this information about your group, I would consider a rest:work ratio of 1:1 (or even less rest between stations) for a more advance fitness level of participants to 2:1 for a less fit group. As for the time spend in each station, again this is up to you and the intensity you want to establish for your program. You will get different answers from everyone and it will interesting to see what others think.
I hope this helps.