Coasting in quiet will take about 6 months. Look at the link.
The minimum thermal should probably be based on size, not crew.
How long it takes to close with a target without burning, and therefore how feasible it is to approach "dark" is tech level dependent. Aurora has pretty high tech levels! If you can reach 1000km/s with a short burn, which is not considered at all a fast speed in Aurora's current setup, it'll only take a day to get to Mars from Earth assuming closest planetary alignment. That six months figure is making very different assumptions about tech level and acceleration than this game does. (I should note that the realistic thermal signature of rapidly accelerating to 1000km/s using rocket propulsion would be visible from an insanely great distance, but Aurora is not entirely realism-focussed in that respect)
Also, the reason I mentioned crew was that a giant freighter with no life support extended to its storage compartments would have a very different thermal signature to a passenger liner of the same size. Other than engines, life support is probably your next biggest heat source. Some kind of function involving both size and crew would make sense, but I think crew is a good approximator of how much heat a ship is going to produce with its engines offline.
You could get a lot more complicated than that, for example, energy weapons produce crazy amounts of heat, should firing lasers increase the thermal sig of a ship? You could even introduce heat sink ship components which, when activated, would reduce your thermal sig but only until they "filled up", at which point your signature would go up higher than it was before. And so on. Loads of stuff could be done with thermal sigs if there's a good gameplay reason to expand there.
I do like the idea of more complicated detection and "there's something out there, but we don't know how many, how fast, and how far. But it's putting out lots of heat." I also think that 100 sig 100 ships flying close together should be almost as detectable as 1 sig 10,000 ship, I believe at the moment that isn't the case, making big fighterswarms really tough to spot.