40 min. in what 1/8 off-road? I'm not the guy to answer that part but for longest life I would charge it at 100 mah or .1 amps and yes it will take forever, like 16 hours. You could cheat a little and charge at like .15-.16 amps and I would still think you would be safe. I've charged my reciever packs at up to 1C which would be 1.6 amps but that might cut down your run time. I'm sure others will chime in on this for you.