I can't speak for all therapists, but doing this full time means working about 80 hours a week the way it does for just about any other small business owner. After a client leaves my office, I have to update the client's case notes, handle any referral requests that need to be addressed, call back agencies I'm coordinating with, fill-out whatever paperwork the insurance companies have decided is important this month, and, if I'm feeling really energetic, deal with marketing, accounting, and other business-related matters. Most of these things actually can't be done in the office, by the way, because insurance companies don't want clinicians keeping client files in a cabinet that clients can see. So, unless you have an office suite with a file room, that's not happening.
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