Would you have symptoms of small cell lung cancer for 18 months prior to diagnosis, or does small cell grow too rapidly for this to happen?

Depends on the grade. After a biopsy, when small cell carcinoma is diagnosed, it is often given a "grade" by the pathologist (e.g., low-grade or high-grade) based on the appearance under a microscope. There are small cell cancers that can grow slowly, & only be discovered on screening CT scans (not causing any symptoms). Others cause symptoms & spread quickly (high-grade). Not all small cell cancers behave alike.