There is no right answer here. The duration of the registration (certification) process depends on the level of compliance with the IS-BAH standard and the results of the audit.
The standard now requires the closure of all corrective actions prior to the certification. So, the FBO will only receive the IS-BAH registration after all findings are closed.
Then, the duration also depends on the audit report processing time of IBAC.
On average, the certification process take 2-3 months. That is counting from the time AWiAS receives the assignment until the final certification of your FBO.