1) How someone does in private practice is HIGHLY dependent upon work ethic, marketing, geographic area, demand, etc.
2) One should learn about efficiency in billable hours. The legal field is a gold mine for this stuff. Hint: you are not going to be anywhere close to 100% billable hours, ever.
3) I would argue that the majority of psychologists are working about half time, although I am very much in the minority here.
4) There is zero salary. Income= how many hours you work times how much the insurance company pays, less expenses which are far more involved than you'd ever imagine, less contributions to operating budget because income varies and you need reserves, less taxes which are paid quarterly and ALWAYS increase, less retirement contributions, etc. Note: if you have a bad month, your expenses don't change.
5) How much start up capital do you have? How much in assets? Credit score? Spouse who can support you for the period of time you're not profitable?
As to your question about how much to expect: Let's say you have nothing when you go into private practice, no assets, but a 700 credit score. You go to the bank, get a commercial loan for about 10k at around 9%. Office rent is 2k and like all commercial loans it requires 3 years contract and a personal guarantee with a NNN of around $200/month, you get a bunch of furniture for only 1k, a crap computer for 500, bad phone line for 100/month, around $700/month in interest, and you have to live/pay rent/eat.