CFS is the standard flow unit on every USGS streamgage in the country. A small mountain creek might run 30–80 CFS. A storied trout river like the Madison runs 800–1500 CFS. The Mississippi at New Orleans is ~600,000 CFS.
For a given river, what matters isn't the absolute number — it's where today's reading falls inside the river's fishable band. FishCast's flow score tells you that directly: same 320 CFS reading is "perfect" on the Provo and "raging" on a small Catskill freestone.