I don't think it has to be a .caf file or a specific length. I made my sound in GarageBand and it's about 10 seconds in total, so not hugely long but more than just a simple beep, and I first made the sound quite awhile ago when caf was being promoted by Apple but I think any standard audio type that is compatible with CoreAudio should work. Also, after you add your file make sure its Target Membership includes whatever Target will be calling the audio, that might have an effect. So if you are calling it from your LiveActivity Target, the sound may have to have the LiveActivity target as a Target Membership. I haven't tried that on a sound (mine is just a simple Lock Screen alarm) but I know that has an impact on other media.
Topic:
App & System Services
SubTopic:
General