Most similar application uses 200...300k ... if the tube is the "classical" #45.
But ... these tubes are usually old, so grid current (near to 0V grid voltage) may vary, so higher value of grid leak resistor may cause a problem (operating point wandering).
But ... these tubes are usually old, so grid current (near to 0V grid voltage) may vary, so higher value of grid leak resistor may cause a problem (operating point wandering).