Your question is pretty valgue as there are a lot of RTOS's out there. A guess, is to put the function in its own thread and make the thread a higher priority than all others so it would not be preempted. Prioritization can be a tricky business though.
HOW the RTOS you are using schedules threads is very important. Many (most?) RTOSs can't actually guarantee a function's run time; they simply run fast enough that with proper programming you can get close enough. Something that takes over 10ms will almost always fall into this latter category since writing a function to use such an exact amount of time is essentially impossible (hardware interrupts alone on embedded system will add enough variance to cause a measurable difference in timings.)