It can be in multiple sessions, but the session cannot be further than 30 minutes apart. The reason why is the reduction or ceasing of cellular regeneration after 30 minutes. Cells initiate repairs within the seconds, and as the window of continual stressor becomes longer, cells detect less and less stressors to worry about and engaged cooling period. All your hard work in getting the cells to engage just become cold, and now you have to restart it all over again.