Negombo is where most international tourists land and where many spend their first or last night. That transit function shapes the town - it's built around convenience, not atmosphere. The beach strip has everything arriving tourists need. Guests who rate it highest came in with the right expectation: comfortable first night, great seafood, easy start.
The fish market at dawn is the reason to set an alarm. Guests who make it there consistently call it an unexpected highlight - one of the most authentic things you can do in Sri Lanka and it costs nothing. Don't skip it to sleep in.
One night is enough if you're transiting. Two nights lets you do the market, the canal, and the beach properly. Don't push beyond that - the rest of Sri Lanka is waiting and Negombo has shown you what it has.