Once your application for an advance assurance has been submitted to HMRC and processed into their system, you can expect to wait 4 to 6 weeks to hear back from them. They will then either send you your advance assurance or they will contact you with questions.
They work strictly in the order they receive the applications. Chasing them or asking to jump the queue is a waste of time. You are in HMRC’s hands. If they are busy, it just takes longer.
In the November 2017 Budget, there was talk of targets being introduced for a 2 week turnaround for advance assurance applications. Great news if these targets can be met!