I also do 30-45 min of HIIT with running. For me, I do a long run (3-5 miles) 1-2 times a week and HIIT 5 times. I currently use running as my "active rest" day activity.
Just listen to your body because if your tired or under nourished, you'll be doing yourself more harm than good. For example, I wanted to get real fit real fast so I did HIIT everyday with a run every other day. I'm 5-3 and I'm pretty good at eating clean, but with that much exercise it was hard to consume the nutrients I needed. I ended up working myself too hard and going on a bender of not so healthy food. My advise, listen to your body. Sometime less is more.
Good luck! :-)

Claire - just saw this post.. what if you did HIIT five times a week and also did the bike three times for an hour.. is there a downside? Thanks..

Frank, just saw this. That might be a bit much, but it depends on how long your HIIT session is. In general is it possible for you to do HIIT on 5 days, and the bike on Rest days? So one or the other as in: HIIT, HIIT, Bike, HIIT, HIIT, Bike, HIIT....that kind of thing?

Thanks Claire - Last week I completed the accelerate series with burnout and liked the weekends off.. I've just not been making it to the gym to get on the bike - I use to go 4/5 times a week and sit on the thing for 45 minutes watching netflix. I'd be drenched in sweat but didn't notice much difference other than to feel fitter. Much better results with HIIT - which I use to do a lot of and then fell off the wagon. So joining sweatflix has helped. I think I'll just focus on HIIT for now and try to do it five days a week. I've just started another series that takes me about 20 minutes a day - but I'm not sure I get the series on the site that are so short - 5 to 10 minutes a day.. Do they actually work? 20 -30 minutes fits into my morning well.

What does the research say about optimal HIIT session length or times per week? Thanks in advance!

Hi Sasa, if you want to see the muscle you have been building, then yes. You will lose body fat if you do some cardio, but I wouldn't do a lot. If you do too much cardio, you will burn out and not have energy for weight training, and you also will lean out quite a bit, and not see the shape you may want to see, if you are doing your resistance/weight training regularly.

I would suggest doing high knees before your HIIT session as a warm up, and alternate those with lunges (or star lunges, or any kind of lunge). This will both burn fat and work the muscles in you your butt and thighs. So I would start every workout with 50 second each of high knees, lunges, then high knees, and lunges again with 10 second breaks in between. That's just an extra 4 minutes of cardio but it will get your lactic acid built up in your legs so that you are burning fat there for the rest of the workout AND for 48 hours after that.

Thank you claire for your quick response :) I mostly do 50 sec of high knees before my HIIT. As you said i'll increase it to 4 mins.