Calm down spot question

We are currently in Module 5 Lesson 9 "Wait for it". We have a three year old and a 16 month old. When utilizing the calm down spot, our three year old often tries to escape as you mentioned Porter used to do. He is very very bright and we feel he could handle a modified rule of three. My question is, would we walk away for 10-15 minutes so he can get calm after he's earned the rule of 3 or do we do the soft hold and for how long? Is there a time limit on that soft hold? He is very very strong and will often start hitting, biting, and spitting at us if we soft hold he often bulldozes right through us. I feel like I have been spending all day trying to get my 3 year old calm and doing corrections that I feel terribly that my 16 month old isnt getting play time with mommy like he should be. Any thoughts and advice is greatly appreciated!