1. How long can the background preparation stage for ISO 27001 Certification take?
I’m assuming you are referring to the time for generating records before undergoing the certification audit.
Considering that, please note that ISO 27001 does not require the minimum period of records (i.e. minimum period of the ISMS operation before the certification), however, some certification bodies do have such requirements and some don't, so you should contact your certification body to confirm what criteria it applies.
2. Can I make my own assessment in this regard without consulting the policies, regulations and expectations of the company directors?
Please note that such assessment is in fact the internal audit, a mandatory requirement for ISO 27001, so you need to perform it, and for this, you need to consult the applicable implemented policies, procedures, required regulations, and expectations of the company directors (these are essential elements to evaluate if the standard´s criteria are being fulfilled).
These articles will provide you a further explanation about internal audit: