Am not the big expert on running a hill, but what i can say is during winter when the track is covered with a few metres of snow, there often is a weekly hill session involved :D
Great thing about hill workouts, there arent any clear answers, so feel your way for what works out for you (But: a general rule is try to have around 20-25 minutes of effective interval time). Bring a heart rate monitor with you, and check: is your heart beating quickly, and feeling out of breath? great. Are your legs starting to weaken? that usually means you are doing something right. For longer workouts, if you can find a hill with 50m climb, that sounds like success to me. (Try also a hill with a smooth climb)
How long should I allow between intervals?
Rule of thumb is the time it takes you to get down the hill again. Thats why if why if you can find a pair of stairs down again, or similar shortcut, then youve found a good place to do some hill workouts. usually you will be recovered enough by the time you get down, so its not really a limiting factor.
How many intervals should I do for a proper session?
As many as it takes before youve gotten tired enough, that you wouldnt be able to do one at the same speed once more. If a person feels after the interval sesion he could probably do one more, he should probably htfu and do another one.
Here's an Example hill workout
. Its a good place to do it, because of the gradual climb and a sweet set of stairs to go down afterwards. In this case its not more than 6% incline average, but it still does the task.