From what I could do testing-wise, in space holding Afterburners I couldn't drain my fuel much in level flight before it recharged after reaching max speed. HOWEVER In atmosphere it gives no shits that you've reached max speed and you can continue to hold Afterburner for as long as you want and constantly drain fuel, I did not test the atmosphere fuel regen yet but the cause of my crash landing was to much Afterburns and not enough rest time.