Add on Psychotherapy codes 90833, 90836 and 90838

We are looking for clarification on understanding of time/minutes for the 90833, 90836 and 90838.  Per the CPT code book a 90833 is 30 minutes of psychotherapy with patient when preformed with and E/M service.  We took a few trainings a few years back when the 90853 med management and 90805 med manage w/ psychotherapy codes were removed and were informed that we could bill a 90833 for any psychotherapy provided outside of the E/M time for time spent between 16-37 minutes. i.e Prescriber did 99213 E/M services then provided 18 minutes of psychotherapy so they would bill the add on code of 90833 in addition.  What time/minutes do you associate with the add on psychotherapy codes?  We understand that no one can advise on what to bill just want clarification on the minutes or span of minutes for each code.