Length depends on what type of cardio you do. If you're doing low intensity sustained state, you may need to do it for 30-45 minutes, but high intensity interval training sessions can be as short as 12 minutes, and are usually no longer than 20. You won't need to do cardio every day.
Cardio comes after weight training. Some people choose to do a very light warmup before their weight training (five minutes at a low intensity) to get the blood flowing, but I personally prefer doing warmup sets or stretching instead.