It would be very useful to display the estimated flight time of crafts somehow, especially in mods which have slow crafts and missions with despawn timers counted in hours or days (and missions which despawn even if targeted).
This would make it easier to try to figure out if the craft will get there in time. (You can return to base immediately if you're not happy with the result.) This might also help if you're trying to figure out whether it's day or night on the other side of the globe when you arrive there, depending on which you prefer for the mission.
While the speed of craft is not linear due to different values of craft acceleration, for simplicity the estimate could ignore the acceleration and be based on a simpler linear calculation t=s/v (remaining distance over maximum velocity). Especially for missions where this would be most useful this would be a very good estimate.
Without bigger changes to GUI I don't see an easy way to display this information when you're selecting the destination, so I'm not proposing that - but rather an easy way to check it afterwards. So, I have two specific suggestions where to place this information.
1) in craft interception overview. Where it now says "EN ROUTE", there's some space to add a few bits of information. This could say for example "EN ROUTE (13.7 h)". I would propose using only the hours here, and the accuracy of 0.1 or 1 hour (how it's rounded up/down doesn't really matter). I suppose the estimate is close enough so that you don't need a "~" there. Omit brackets and use just hours without decimals if you need to conserve space. So the most minimal way to display this information would be, for example, "EN ROUTE 2h".
2) in craft destination details. Where it now says "STATUS>(...)", if the craft has a destination, add a row below saying "ARRIVAL IN: 13.7 h.", with yellow and indented to the same level as DESTINATION. If you want, this could also show days, hours and minutes instead of just showing it in hours. ("ARRIVAL" or some other term instead of "FLIGHT TIME" to use a neutral term that applies to all kinds of craft.)
Instead of a new row, it might be possible to add the information at the end of the destination string like in 1), but this might lead to problems if the destination string is long. And because there is lot of free space in this dialog, I'd propose adding a more detailed information as a separate row.