Your question is very important but has not an easy answer. Timing for implementation is a function of resources used in the project, people involved, their knowledge, and time for the project. You need to work with three variables: What to do, who will do it, and when to do it.
Let’s go first to the “What to do?”:
decide the scope of the quality management system. Is it applicable to all areas and services or just for some?
perform a Gap analysis, to determine the amount of work to be done - comparing what your organization already has in place versus ISO 9001:2015 requirements. From that GAP Analysis you can develop your Project Plan, listing what needs to be done
from there it is the implementation in order to close the gaps found. Then, perform an internal audit and the management review. There you can decide if your organization is ready for a certification audit.
Then, who will do it?
Setup a project sponsor, a project manager, and a project team. Ensure top management support, get training about the standard. Designing and implementing a quality management system implies being knowledgeable about ISO 9001:2015
I see some organizations taking a long time because they don't provide a project manager with enough time for the project
When to do it?
It is a function of the commitment and time of the project participants