Ok, so I have a couple questions about cardio. The first is about my personal cardio, and the second is about cardio in general, so here it goes:

(1) My cardio consists of a 5-day-a-week plan, running with high intensity. I read in Men's Health magazine that calories continue to be burned at a higher rate if one ran hard the first half of the time, and then the second half, ran at a easier pace. (As opposed to running for an extended period of time, going slowly, and picking up the pace towards the end) Based on this, I have been running on a treadmill for 20 mins a day, on this basic plan:

Minutes 1-5: 7 mph pace
Minutes 5-9: 8 mph pace
Minutes 9-10: 9 mph pace
Minutes 10-11: 8 mph pace
Minutes 11-19: 7 mph pace
Minutes 19-20: 9 mph pace
(5 minutes cool-down, going from 4.5 mph, to 2.5)

Is this effective for continual calorie burning, and in turn, noticable fat loss? If not, could you recommend a more beneficial plan?

Ok, on to the second question...

(2) What is an appropriate amount of cardio? I had heard that, if performed at a challenging/effective pace, that 20-30 minutes is plenty of time. I have seen people's plans on JSF, however, that people are running for half an hour in the morning, half an hour at night, and more if they have a chance. That seems like a bit much to me, and might just over-exert one's body.

Any ideas on what's best for weight loss?

Thanks!

You're gonna get mixed answers on this subject because there's not a single thing that works best for everybody.

For me, the best fatburning cardio has been 45-60 minutes of low-med intensity cardio in the morning. I did 23 mins of HIIT for a few months and it was working, but after I decided to give the low-med intensity cardio a try, I just noticed that it worked alot better.

I would just grab a few of the cardio workouts you like the most and then try each one for 2 weeks while monitoring your weight/bodyfat very closely. See which one works the best, and then go from there.

In my opinion, the best cardio you can do isn't "subjective," in that what you perceive as "hard," "high," "low," etc. It's all calculated by your heart rate. Calculate your maximum heart rate (easy way is 220 minus your age). Then take 65% to 70% of that to get your target heart rate. Train in there for 45 to 50 minutes. I think this is the best way to do cardio.

On a side note, I don't really trust too much I read in Men's Health or Men's Fitness. Sure, they have some good articles from time to time, but they also fall victim to publishing the latest fad of the week, from diet to exercises. I think you'll find that high intensity interval cardio is last week's fad. So are all those 1-month lifting routines for "cutting up" specific areas. Your lifts should be the same during bulking and cutting--just your calorie intake should be different. Also, these magazines like to use isolation movements to "cut you up." I still think the best lifts are good ol' fashioned compound movements with heavy weights.

"Is this effective for continual calorie burning, and in turn, noticable fat loss? If not, could you recommend a more beneficial plan?"
If, through cardio, your goal is to induce the highest level of EPOC (post workout calorie expenditure) then the more intense your cardio, the better. In my opinion the general guidlines for H.I.I.T ie. 1minute of high intensity (above 85% of max. Heart Rate) followed by 1minute of low intensity giving you just enough time to recover before repeating the process, can be bettered if a higher EPOC is desired, I will elaborate:

As i stated, the higher the intensity, the greater the EPOC; therefore if a high EPOC is indeed your target then your goal should be to keep the intensity of the work interval at your maximum, for as long as possible (usually 10-20seconds), and then move to the rest interval when there is a need to slow down at all in order to recover just enough to go all out again.
As a test, sprint the 100m's as quickly as possible, you should maintain this intensity throughout every work interval (10-20seconds).

Be sure to get enough rest between the work intervals as you will not be able to give 100% on them without it.
So...depending on your fitness level and the time you need to recover, you could try this:

4 mins warm-up (light to moderate jog)
50 second Jog / 10 seconds All out
48 second Jog / 12 seconds All out
46 second Jog / 14 seconds All out
44 second Jog / 16 seconds All out
42 second Jog / 18 seconds All out
40 second Jog / 20 seconds All out
* then reverse it
40 second Jog / 20 seconds All out
42 second Jog / 18 seconds All out
44 second Jog / 16 seconds All out
etc. etc.
Finish with another 4 minute jog = a total of 20 gut wrenching minutes which should leave you pretty wacked for a while afterwards.
Once you master this, increase the work intervals and decrease the recovery time (never allow the work intervals to suffer if you're serious about increasing EPOC).
Do this only 3 times a week as your body needs time to recover.
Make sure you fuel yourself proparly as glycogen is the main fuel source utilized, no glycogen in the blood, wave goodbye to muscle.
Treat this like most would treat their weight training, Low G.I carbs/Protein an hour or so before, high G.I carbs/Whey drink afterwards.
Do not do this on your weight lifting days whenever possible.
*Either do this outdoors or on a machine that allows you to go at full intensity (not a treadmill).

Any ideas on what's best for weight loss?
Regarding what's best for FAT loss there is a lot of debate here. It seems that H.I.I.T done correctly (including proper nutrition) is more muscle sparing but over-training is more likely. While moderate intensity cardio done for 45-60mins, in comparison to a 20 minute H.I.I.T session, burns more fat directly and is likely to burn more calories during the course of the exercise. *It seems that total fat oxidation (amount of fat burned) between the 2 tends to even-out by the end of the day due to the EPOC of H.I.I.T. Personally i would alternate your cardio sessions fairly regularly as i believe both have their place. Give my suggested H.I.I.T routine a try for a few weeks and see what happens.

Don't do more cardio then you need to see results because when you reach a plateau, or results slow, you can always add more cardio rather then dropping calories (resulting in a higher risk of muscle loss while simoultaneously creeping closer to the level at which the dreaded "starvation mode" kicks in).

Keep us posted on what you decide to do and your progress doing it. :gl: