I've come up with a few interview questions, and reviewed a few more. In my experience there's a ~decent rule of thumb for gauging difficulty and time required

I've come up with a few interview questions, and reviewed a few more. In my experience there's a ~decent rule of thumb for gauging difficulty and time required to solve a given problem. A person in your company, after having read the problem and glancing at the rough description figures out a clean solution in time X. That's without the interview stress, knowledge of the problem domain, with the pacing of the problem laid out in writing. A well-performing candidate coming in cold, stressed from the interview setup and under time pressure will take 3X time to solve it. So if it takes much more than 10 minutes for someone at your company solve your proposed interview question reasonably well, it's almost certainly too difficult.