Falcon heavy launched at 3:45
Musk tweeted it reached asteroid belt at 10:46
So call it 7 hours (25,200 seconds)
Distance from here to ceres orbit is 414 million kilometers
414 million km / 25200 seconds = 16,428 kilometers per second
Speed of light is 299,792 km/sec
Just back of the envelope calculations, unless I did something wrong