In the real world, a straight departure typically means you stay within the limits of ±15 degrees heading from the centre line of the runway. So if you depart from runway 27, runway heading will be 270. After takeoff any heading between 255(270-15) and 285(270+15) is a straight out departure.
Clarification – June 28, 2019: In Infinite Flight though however, to simplify things, a straight out departure simply means aircraft should maintain runway heading until no conflict exists. When in doubt, fly straight out.
1. 2.1 from the International Virtual Aviation Organisation (PDF)
2. 7.1.6 of the ATC Manual (PDF) retrieved on June 28, 2019.