Depends on the source. Some streaming companies automatically stop the stream after some period of time. Running from local service, such as an NAS or a hard drive rely on that device being turned on, and always accessible, they would run as long as the playlist, or as long as the repeat button is engaged, forever. Any stopping then would be a network issue. 

Okay, I was using Sonos HD radio when it happened

I think it may depend on the station source there. Not sure they’ve published a “this will go for X time” for each individual station, and not all stations are not “owned” by Sonos.

If I were to guess, it’s likely a wifi interference event, but it’s hard to tell based on just a verbal description. The next time it occurs, submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.