Start as soon as possible.  The credentialing process can be lengthy depending on when you hit the insurance carriers credentialing cycles and they are all different.  Typically applications are processed within 90-120 days, however, I have experienced a carrier that has taken a 1 ½ years to become impaneled with due to their credentialing cycles.  It was very frustrating to say the least.
 
Details Matter.  Most clinicians don’t like to do their own credentialing because it can be very tedious with all the details.  When completing applications details matter, just like in 1st grade, follow the directions.  The boxes and checklists are on the application for a reason.  Complete every section and make sure you enter the correct information, for example:  know which NPI is being asked for, is it the individual NPI or the group NPI number.  Leaving a box incomplete or left blank can hold up your application or cause it to be rejected.  Due to the insurance companies merging over the past couple years, the provider reps don’t always have time to call you for clarification on whether your service location is ADA compliant or double checking dates on your employment.
 
CAQH is up-to-date.  Many times the CAQH (Council for Affordable Quality Healthcare) is the first stop for insurance companies when processing your application.  It must be current and accurate, otherwise they have to track you down for the correct information, which is of course causing a delay in your application, thereby delaying your acceptance to panel and delaying when you can see clients and get paid in-network reimbursement.  Staying current also tends to make re-credentialing a breeze