This has been discussed many times, and it depends how well (or bad) the scenery AFCAD is made.
In order to have service vehicles arriving from away doing a realistic route, without clashing into AI airplanes, GSX give a much higher preferences to these routes, even when they are longer and, it tries to spawn the service vehicles from a vehicle parking, even if it's far away.
Previous versions used to put a limit on the maximum distance from your parking spot, in order to not have to waiting too much, but it resulted in problems when the airport didn't include any of such spots, and all the airplane parking spots (which will be used as a fallback), didn't had any connection to any vehicle route, so we were forced to use taxiways, clashing into AI, and people assumed it was a GSX bug, rather than a sloppily-made AFCAD. So we fixed it, by not making any distance limit to search for vehicle parking spots, since if they are found, they are usually connected with a proper vehicle path, which is always the preferred route to not interfere with AI.
Of course, if the airport is well made, with multiple vehicle parking spots correctly placed near all major terminal, as in real life, all connected to the gate with proper vehicle paths, waiting too long wouldn't be an issue, because that's the same reason why vehicle parking spots are placed this way in real life...
