Everything, it seems, works in cycles and your workouts should too. It is a good idea to modify your workout routine in terms of length, intensity and activity in 2 to 3 week cycles.
Every two weeks, I like to vary my workout in terms of duration and intensity since I try to mix up my cardio methods weekly. So for 2 or 3 weeks I work at a lower intensity for a longer period of time, like for 40 to 60 minutes. I like taking hikes in the hills, jogging in flat areas, or swimming at a steady pace for a long time.
I follow that cycle with a couple of weeks of more intense workouts. I take to the hills for my runs or head to the track or pool for intense interval training. These workouts I limit in time to 20 or 30 minutes. If I have my wits about it, I can organize my exercise phases with how hectic my schedule is going to be. Short intense workouts are great when I have out of town guests or a busy social calendar - 2 kids in preschool means lots of birthday parties and pot lucks.
Another reason this works for me is variation of any sort prevents both my brain and body from getting bored. Boredom often leads to folks falling off the exercise wagon. This cycling of intensities helps the body become more efficient at burning fat. And what is not to love about that?