I suspect the question may be a trifle subjective; dependent upon the nature of the propulsion system, the energy levels, as also the nature of the object impinged
The exhaust from a space-craft is very high velocity (Wikipedia says as high as Mach 10. In atmosphere (perhaps as high as LEO) the exhaust probably loses energy relatively quickly, or at near distance.
When travelling in deep-space, how long/far would it take for the ejecta to lose energy? How closely could space-craft, as depicted in popular literature, 'fly' in 'formation'?
Say, a space-craft is under assembly in orbit, how closely could an astronaut equipped with an MMU follow his/her/it's buddy?