If the ATC tells me to head 270 for 50 I head bearing 270 for 50 miles.
Unsure the best way to tell how far 50 miles is, but I set the HSI to the runway bearing and hopefully your waypoint is on the airport, so it will start to line up when you near 50 miles and I begin to turn towards the airport.
Is there a better way to tell when you've travelled 50 miles?