A launch from Kennedy (KSC), seen from Washington, D.C.
From Washington, D.C., Kennedy Space Center (LC-39A) is about 1191 km away to the SSW (compass bearing 197°). A night or twilight launch is unlikely to see; a daytime launch is unlikely to see. Face SSW shortly after liftoff and watch low on the horizon — the rocket climbs as it heads downrange.
You're not watching the rocket itself — beyond a few km the vehicle is far too small to see. What's visible is the engine plume at 65–100+ km altitude: at night a brilliant moving flame (routinely seen 300+ km away), at dawn/dusk a sunlit "jellyfish" cloud that has been documented 800+ km away, in daytime only a white contrail with much shorter range. Cloud cover, local horizon and trajectory all matter.
Nearby cities
For exact launch times and live trajectories, check the provider's stream. How this estimate works.