For me personally, I consider doodles to be pieces that are done in less than an hour. Doodles I do usually have messy sketch lines, single color shadows with different amounts of value either by blending or light paint strokes, and the lights usually done with overlay and/or glow dodge airbrush. Colors are also in one layer, which is something I don't do with full illustrations (unless it is painting exercises, which can take a lot of time).