DO NOT use sim time at all for this achievement!! It will void your progress.
This seems to no longer work with the CJ4. I'm sure it did on PC before the recent updates, but now that the game is running on Xbox this no long works on Xbox or PC. It must be a propeller or turbo-prop aircraft. I tried multiple times to get the Frequent Flyer Miles achievement and it took about 15 hours with the CJ4. You can use sim time on that flight and get it over with, but DO NOT use sim time at all for this achievement!! "Frequent Flyer Miles" can be sped up, but not this one.
I flew CYHZ to LFPG with the Daher TBM 930 and it took about 8.5 hours. The AI landed the plane in a small pond and continued to the edge of the runway, but it still counted. I chose a gate for both airports just in case that is needed. (Image #1 below)
I used the AI pilot the whole time. On Xbox, click the left stick to bring up a cursor or use a mouse. In the toolbar at the top of the screen is a pilot head icon that says "Flight Assistant". Turn on AI Piloting and use all assists in the options menu. (Image #2 below)
When you start the flight, the plane will be off. Even with assists, I had to turn it on. Click on the Checklist in the toolbar and the categories for "Before Engine Start" and "Engine Start" will need to gone through. The bottom of those checklists will have an autocomplete option. (Image #3 below)
Once the plane is started, the AI will start to taxi the plane and the flight will start. Check back in 8.5 hours for your achievement!
You can go with just about any two destinations that show the flight time of 8+ hours, but I want all the star airports and needed LFPG. The TBM 930 is the fastest plane you can use, but 8 hours is 8 hours. The plane does not matter. This plane will take 8 hours for the flight path I used. A slow plane will take a lot longer, so pick two that show a total time of 8 hours.