Drying time is determined by the volume of air moving through your dryer, also how thick your meat is sliced.
Preference - how wet do you like your biltong, biltong always going to be substantially wetter on the inside than the surface of the meat. Meat will feel hard to the touch on the outside and soft when depressed. Meat should feel soft throughout.
My suggestion would be to leave the light (heat source) off for first 24 hours. Let the fan do its job. temperature in box should be 28 degrees. I would not use the light bulb if air is dry, I would recommend only using fan in humid climates. Then again many of us are impatient and a light bulb speeds up the process, I don't like the end result, light bulb makes box far too hot, meat hardens (cooks) on the outside.