More speed than potential hostile ships. I would say 100 m/s minimum, more is obviously better. Problem is, that many combat ships are quite fast.
The best you can hope with a cargo ship is to get close to the speed of hostile combat ships. This won't allow you to run away in a straight line, but it will allow you to deny them the complete control of the situation by trying to get out of their line of fire (not away from them).
Things get complicated at this point as now turn rate and acceleration comes into play.
If you fly a cargo ship you won't outrun a "meta murderhobo", because that's the whole point of such a meta ship. All you can do is to buy enough time to get out by not allowing the attacker to constantly be in the perfect shooting position.
