I'm planning my own fitness plan and want to add some HIIT. I have lots of weight to lose (100+ lbs). How much HIIT is too much?

Per day? Per week? If you are really doing HIIT, it's pretty tough to do more than 30-35 minutes even if you are in shape. Remember, the exertion level is perceived by you. I generally tell clients who are healthy but need to lose weight to do HIIT 3 times a week and add one or two sessions of "regular" cardio. Are you doing any other exercise or just cardio?

Mel

When I was doing HIIT, I had to work up to it. I usually only did a few minutes every time I did cardio (I would try to do it in the middle). It seemed to take a long time to work up to regular, somewhat extended HIIT. But it's very effective so add it, but slowly.

Excuse my lack of education on this but what is HIIT? High interval intensity training? lol that was just an educated guess.

I think it actually stands for High Intensity Interval Training (darn good guess). Basically the idea is that (after warm up) you go as hard as you possibly can for a short period followed by a moderate pace and then more high intensity, back & forth mixing it up.

Oops .. I did fail to elaborate. I have mainly been doing cardio and wanted to add weight training and HIIT. So, working out 6 days a week, I just wanted to make sure I wasn't overdoing it. I think I'll start slowly like Susanje suggested and gradually work my way up to doing it 3-4 days a week, 20-30 minutes at a time. Does this sound reasonable?

I like ... this article about HIIT ... I like HIIT a lot because it just feels like I've worked harder...