Stakeholder management: What is the reasonable length and is there such thing as ‘stakeholder creep’?

It is not unusual for the number of stakeholders to increase throughout your project. Reach out to your marketing and public relations departments to get as exhaustive a list as possible from the start.

I wish there were a standard formula like the number of communication channels . Unfortunately there is not.
You start your project with a base set of stakeholders - sponsor, key managers, key subject matter experts and keep adding as you progressively elaborate your scope.
And then you finish your project and realize that the end product does not meet the customer need because you forgot a key stakeholder :) .
Quote unquote from The Matrix by Morpheus
"WELCOME, TO THE REAL WORLD ".

Time ago I wrote an article that was published by the PMI. Here the link with the hope that it helps (https://www.projectmanagement.com/blog-pos...h-stakeholders)
Talking following the PMI there are two roles today: business analyst (BA) and project manager (PM). Projects are started to create a solution. BA starts working before a project exists to help the organization in defining the solution and PM starts working when the project has approved. But as best practice both roles must work together along the journey except the point where the project ends and BA continue working to monitor if expected benefits are achieved. So, all related to stakethoders begins at zero moment when the need to put strategy into action emerges and is the BA who starts working on stakeholder management. PM will add steholders more related to project working with the BA. The length could be hugh but the stakeholders into the radar are defined according your solution team structure which is on hands of BA. Solution is equal to "the thing" to be created plus "the process" to create it. Just to clarify, if you use PoC (point of contact) structure (the person who is the ambassador for a group of people or business unit) that is the people who have to list into the stakeholder list (for example, BPoC - Business Point of Contact, TPoC - Technical point of contact, and so on).

Stakeholders management - as many other PM areas - must be live. Plan, execute and review until the last day!

I agree with Kiron and Keith on this one. Stakeholders can change over time although Sponsor an key stakeholders should generally stay fairly fixed.

The interesting part is the "Stakeholder Creep" in an organisation like yours and others. Although you wont find this in any book (correct me if I'm wrong anybody but I've never seen it) it actually does exist. After you've mapped out your stakeholders and you have your list of Key stakeholders and set up regular meetings with them, after a few weeks or months you find that key stakeholders that haven't been able to attend (or can't be bothered) and sent representatives have kept the rep's on the distribution and they attend regularly with the key stakeholders. So instead of a quality group of 8 key stakeholders you have 14 or 15 at some meetings and 4-5 at others with a distribution list of 20 all of whom want their voices heard.

This should be manageable by setting up the meeting ToR at the start and what is expected of the stakeholders and managing the distribution list keeping only key stakeholders on the list unless there is a proposed change.

And as Deepesh said "Welcome, to the Real World"

Stakeholder management is an ongoing process and you have to update the list of your stakeholders and their needs.

I concur with Kiron on this.
It should be a point of focus for the project manager, also has to assure to maintain positive and valuable stakeholder relationships.