From X-Plane SDK
XPLM_API void XPLMSetFlightLoopCallbackInterval( XPLMFlightLoop_f inFlightLoop, float inInterval, int inRelativeToNow, void * inRefcon);
This routine sets when a callback will be called. Do NOT call it from your callback; use the return value of the callback to change your callback interval from inside your callback.
inInterval is formatted the same way as in XPLMRegisterFlightLoopCallback; positive for seconds, negative for cycles, and 0 for deactivating the callback. If inRelativeToNow is 1, times are from the time of this call; otherwise they are from the time the callback was last called (or the time it was registered if it has never been called.