It used to be part of my job.
BLUF (bottom line up front) Have your clinic do it.
There's always fine print associated with every clause & proviso in an insurance document. Essentially every single number or statement on the patient information sheets? Has between 1-5 paragraphs attached to it in the actual policy. What you'll want to request is a copy of the actual policy. If the copy you have is less than about 100 pages (could be as little as 50 up to several hundred pages) in very very fine print? You have the abbreviated version.
Working with a TaxID (aka medical professional request), it generally took me between 24 & 72 hours to get the relevant documents from the insurance company. As a policy holder? 6-12 weeks. And then I t was usually he wrong info, and I had to submit another request. Insurance companies draaaaaag their feet in sending useful information. So DO try and 1) get your therapists office to get preauthorization for you (it's much faster... and easier, since they're the ones doing it ;)), or 2) Don't ask. But instead download a copy of the whole f*cking thing yourself from online, if you have access. ((Insurance companies are pretty notorious for removing patient access to their policy once they start challenging it.))
Once you have your actual policy in front of you there are 2 places you want to look: the obvious (psych benefits) & the sneaky (outpatient benefits).
In both places there will usually be adjusted rates & numbers & maxes for continuing past the allocated number of visits per anum. You'll want to highlight the paragraphs that relate to what you're looking for, including the billing codes, and form ID numbers (because every possible request has a specific form). That form will have to be filled out by your healthcare provider, or they'll need to give you the info so you can bill your insurance yourself (TaxID numbers, provider ID numbers, etc.).
So first you fax/email the preauthorization form, (don't mail it) with all the info. Then they usually deny it, proforma, and you submit an appeal (different set of forms), and they either accept it (usually the case, the denial is just to make policy holders think the actual answer is "no", while professionals just resubmit everything, because the "no" is meaningless).
Professionally, that whole process takes about a week.
As a policy holder? It's generally about 6-9 months. Unless I've got access to OneHealthPort and a few other databases I can attach reference IDs to bully them with. In which case it's about 1 month.
Really... Have your clinician's secretary do it.