If you want to see the shock front move on something like a second timescale, I'm pretty sure you need to be close enough to be in real trouble. Typical expans

If you want to see the shock front move on something like a second timescale, I'm pretty sure you need to be close enough to be in real trouble. Typical expansion velocity is on the order of 10 000 km/s radially, so you get 20 kkm increase in diameter per second. For safety then, 20 kkm should correspond to a barely perceptual angular change, which wikipedia tells me is around 1 arc minute. So if you are close enough to just see the shock front move, you have about an hour until it gets to you, and if you want to avoid being overtaken you need to be able to move at ~ 10 kkm/s.