It really depends on your comfort level, but I can tell you this--modelers who look for a "pat" method of weathering--you know; 1. paint, 2. add filters, 3. pinwashes, 4, add pigments, and so on and so on--are missing the point. Weathering is not a destination, it's a journey where the destination arrives without you knowing it. Kinda like where Jeremiah Johnson finds that place along the river and decides that "We'll live here"--you know it when you get there, But sometimes you can't "get there" with a map; you have to break a few branches to find the promised land.
I pinwash at least twice--the first time for definition over the base coat, and then a second, or even third pass to really add shadows and definition. You have to make exaggerated definition in 1/35 scale in order to simulate the way that light falls on a real vehicle-the so-called "Scale Effect". Add some early, and then always keep the possibility open for adding more later. When you put down pigments or even an airbrushed dust coat, you'll lose some that definition you had the first time. Don't be afraid to go in there for a second pass, or even more if you think it could use it. Take your time, weather in layers, and don't be in a hurry to get something done. That's the secret to modelling magic.