Much of that answer depends on your fitness level. A person who has been mostly sedentary will need to start off lightly, while one who's been very active can push themselves intensely. It also depends on what results you want. If you trying to get rid of fat, cardio, provided you do it at enough intensity, works best.

When I started my plan weighing at 240lbs, I began with two 30 minute sessions per day, and each week would increase both sessions by ten minutes. Each week, I would lose 6 pounds, until the 7th week, which I only lost 2 pounds.
The following weeks, I did only one session of cardio and increase weight training intensity.
Nowadays I do weights, rest, cardio; rinse, repeat.