If your clients are in good condition and are not beginners, 2-3 days in a row would be ok. Most of my clients can do HIIT 3 days in a row and they have no trouble performing at a satisfactory level. I also have clients who can train at this intensity level for 4 days in a row with no trouble. As Susan mentioned, it depends on the participants and their goals. Everyone adapts and responds differently to training, so you need to know their limits when you are designing their program.
I hope this helps.
It all depends on the duration and demand
I personally do HIIT twice a week,
If I were to put it in two or three days in a row I would make sure each session was totally different and there were rest periods.
This also depends on the condition of your client and what their goals are.
Not everybody responds well to everything!