Hi,
Average credentialing takes anywhere from 2 to 6 months. It really varies from carrier to carrier. I personally feel that if you plan on participating it is better if you start out that way. Most carriers start your participation date from the date you submit your application, or the date you indicate on the application that you will be opening. For example, if you submit your app with Cigna and indicate you will be opening Sept 1st, they may not finish the credentialing until Sept 15th, but they put your effective date down as the first.
BCBS goes by the date you sign their contract. You have to start the credentialing process, then they send you contracts. Your participation effective date is the date you signed (at least NY BCBS).
You are also going to need to do CAQH apps (if you haven't already) for each therapist and those each take time. Many of the carriers you mentioned require the CAQH apps.
If you know when you are opening (approx), the address, and the therapists then you may want to start contacting the carriers now to get some of the paperwork moving.
Good luck
Michele