Either work it out prior, add it to an array. Like a grid of cities (like the pokemkn effective types chart)
Or if you have one giant map with the cities on. You can have text say
"time to city"& (distance (city A.x,cityA.y,cityB.x,cityB.y))/speed
This should give the time estimation... for a straight line...
If you have roads, i would advise getting a path finding bot to run the route, set a timer, and stop it when it reaches the target city. Write this down lol