Assuming I'm not using a computer program to generate a flight plan for me, for cross-country IFR flight planning in the United States, how quickly after finishing departure procedures is one supposed to get into an airway? To generalize the question a bit, I'm trying to understand when using an airway is required and when it's ok to route to waypoints outside of airways.
As a sim hobbyist one of the things I enjoy is learning about charting and route planning.
UPDATE: In addition to answers provided here, I also found it helpful to read Instrument Procedures Handbook, FAA, Ch 2, En Route Operations.