Hi! I'm currently in the midst of researching / using this code for my practice. This is a time table I pulled from my state's Medicaid portal (Wisconsin). I don't know if you need all this info but, maybe it will answer your question.
8-22 minutes 1.0 unit
23-37 minutes 2.0 units
38-52 mins 3.0 units
53-67 mins 4.0 units
68-82 mins 5.0 units
83-97 mins 6.0 units
and should also include the following modifiers depending on which kind of service was used
U1 - spoken language (the interpreter is physically present)
U3 - sign language (the interpreter is physically present)
GT - audio/visual telecommunications (the interpreter is NOT physically present)
93 - audio-only telecommunications (the interpreter is NOT physically present)
Interpreters CAN NOT be a family member of any kind: spouse, sibling, etc. that is present with the patient. Interpreters can only be a qualified interpreter. For the state of Wisconsin, we do not have requirements for foreign language interpreters but, there are licensing requirements for sign languages. These requirements will vary state-to-state Medicaid programs.
As for provider documentation, they have to document the time spent in the appointment and note that the interpreter was present and interpreting for that time. Time spent outside the exam room can not count (such as checking for the appointment).
This is strictly for Medicaid. I don't know anything about commercial insurances or Medicare at this time.
Per HCPCS code description, the T1013 is per 15 minutes so, I am assuming that if 30 minutes was not met than you can only bill 1 unit. It would be worth checking with your state's Medicaid and see what they say!