What is the recommended period for having injection timing checked and / or adjusted?

If there is a recommended interval I'm not aware of it. If you are changing oil regularly such that you don't get timing chain stretch, there's really nothing that should make it change.

If you're not changing oil often, the microscopic particulate that circulates in the oil shows up as wear on the timing chain. If it runs well, I wouldn't worry too much about it.

My 240D went 380,000 miles without it ever being checked. I would not have touched it then had it not been overhaul time.

Good luck,