At the end of Book 6 of my book series, a magic ritual has caused a large meteorite (around 1/4 the size of the one that caused the chicxulub crater) to ram into the small planet (around the size of Earth's moon). However, the main characters are some time away from their ship when the meteorite hits. If the meteorite hits the opposite side of the planet, how long do these characters have to escape before they are swept away by the shockwave?
Rough estimation: the shock wave is basically a seismic wave.
The fastest seismic wave (P wave) travel at about 5.5 km/s in granite. Considering that the diameter of the Moon is about 3500 km, and assuming that the propagation of the seismic wave goes along the shortest path and with uniform velocity, we have about 636 seconds, or 10 minutes, before the shock wave reaches the other end of the body.
Feeding "1/4 the size of the Chicxulub impactor" and "other side of the Moon" into an Earth impact calculator gives an approximate time of "forever", even with worst-case estimates for the size of the meteorite. Key numbers from the calculation:
The fireball is below the horizon. There is no direct thermal radiation.
The major seismic shaking will arrive approximately 18.3 minutes after impact.
Mercalli Scale Intensity at a distance of 5500 km:
IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.
The ejecta will arrive approximately 28.8 minutes after the impact. At your position there is a fine dusting of ejecta with occasional larger fragments
The air blast will arrive approximately 4.63 hours after impact.
Peak Overpressure: 5400 Pa = 0.054 bars = 0.767 psi
Max wind velocity: 12.4 m/s = 27.8 mph
Sound Intensity: 75 dB (Loud as heavy traffic)
If your explorers are out wandering, they probably won't notice the seismic effects. Instead, the first evidence they'll get of the impact is a dusting of powdered rock ejected by the impact (I suspect the actual arrival time will be closer to 45 minutes than the given 28.8 minutes, due to the smaller planet size changing the duration of ballistic trajectories). About four and a half hours after the impact, the shockwave you're worried about will arrive, in the form of a loud, protracted rumbling.
In short, it'll be obvious something happened, but your explorers won't be in any danger until secondary effects (weather changes, firestorms) start arriving, probably days after the impact.