Is there a rule of thumb for estimating internal labor costs for developing training? i.e 10 hours of design time for every hour content?

For me this is a project by project basis of creating an estimate and then reviewing actuals.

Each project is different. It depends upon whether the training is in-person instructor led, virtual instructor led, eLearning CBT, blended, etc. It also depends upon what materials you are starting with, SMEs, knowledge of the instructional designer on the subject, etc.

There are some "rules of thumb" but I have not found them to be helpful.

I have worked on projects on which we spent 25 hours of development time to 1 hour of developed training. And others the ratio was 50 hours per hour. It all depends on complexity of the development.

The only way to know is to start tracking. We started tracking our time, like consultants and lawyers. Instructional designers started tracking the hours they worked on each course they were developing. Once we did a few, we developed our own model to project future projects. We got better and better at telling the business how long it would take to complete a project for them.

It is hard to get a designer to start tracking time like that if they have never done it. Consultants and Lawyers do it all the time...so why not designers. Just takes some discipline and many, many reminders.