How long will Google keep old URLs indexed after a change of address?

How long will Google keep old URLs indexed after a change of address?

Ok, so this question is only directed to the SEOs here that dealt with a website migration from one domain to another at least once. 

I want us to share our experiences and maybe find answers.

I personally coordinated a few dozen domain changes over the years, and I simply had no problem with the old URLs, most of them were out of the index in a few weeks. 

But with my last client, I ran into a pickle. 

It's been three months already, and ALL the old URLs are still indexed, and I believe I did everything by the books.

Here is what I've implemented so far, let me know if I missed anything:

• Redirected 301 all URLs from the old domain to the new domain.
  
• The content remained the same, only the domain changed.
  
• The website had only 60 or so index pages, nothing complicated, the structure was straightforward. 
  
• Made sure to submit a change of address in Search Console to let Google officially know about the change.
  
• I moved the old sitemap to the new domain and implemented a 301 redirect. Basically, olddomain.com/sitemap.xml now redirects to newdomain.com/old_sitemap.xml
  
• Submitted newdomain.com/old_sitemap.xml in the Search Console of the new domain so that Google can understand the move. 
  
• Submitted a new fresh sitemap to Search Console with all the new links. (newdomain.com/sitemap.xml)

In the past, all of my change of address migrations were fairly uncomplicated because the new domain was freshly bought and I always insist on a server/hosting change also. 

The current client I'm currently having problems with has the following weird factors:

• The new domain used to 301 redirect into the old one (for at least six months). 
  
• The server is the same.
  
• The IP hasn't changed.
  
• Basically, everything is identical from hosting point and folders point of view, the only thing that changed was the domain.
  
• Olddomain.com/robots.txt is still accessible.

What am I doing wrong? The server/hosting change is off the table. If you think there is something I missed, please let me know. Do you think it just takes a lot more time because of the shared similarities, or will I still be seeing the old domain index one year from now?
What do you think I should do about the old domain robots.txt? Because I've read some messed up stories about people messing with the old robots.txt which in turn affected and influenced their new domain because Google for some reason,n still saw the old domain robots.txt as the main robots.txt and applied the same rules to both domains, new and old. 
Also, how much should I keep the old sitemap around? Because I know for sure, Google recommends keeping it around for a few months. But when should I remove it, and should I just delete it completely? 

Again, if any of you know what I'm talking about and had similar experiences, let me know and let's discuss!