To maintain and support an entire OS takes a lot of work. We aren't talking about just development here, but checking to make sure things run properly and making the changes needed to ensure stuff is supporter. The point I would start looking at rolling your own distribution and supporting it is the day you decide to start selling your distribution.
For internal use, sure you might have to have a team to do internal work to modify certain sections in order to make the OS work for you, but they are relatively minor compared to ensuring an entire distribution works as needed. Let another company do the heavy lifting and just have your company modify it and submit changes back through the system as desired. Feedback works as well.
To run an entire distribution and all the subsystems takes billions, look at IBM donating to Linux as a whole they give value back to the community rather than trying to extend and embrace for their own purposes. Redhat does the same and they do distribution and sales. Other companies are the same. I guess you can make the decision on your own but personally I suppose the time to switch is when you have support fees in excess of what it would cost to maintain an entire distribution. I'd assume someone around a thousand people focused on the project would be about right. A thousand people's salaries would buy a lot of support. A better idea might be to hire developers for the subsections of the OS that you need and have them work with the community.