How long do you wait for the cops?

Kinda long story, but wife, daughter and I were out for late dinner and witnessed a fight between what seemed to be a guy and the brother and dad of his gf (who was said to be 16 by dad - bf was in his 20s).  Dad and brother showed up and confronted him and pushed him around and eventually the brother knocked him down with a left and then dropped a few bombs before taking off in his minivan while dad took off in his car.

Owner called the coos and as we were only patrons were asked to give details to dispatch (owner spoke broken English).  We had finished our meal and decided to wait for cops but after 40 minutes they hadn't showed up.  Place was getting ready to close so we left our contact info and headed home.

Were we supposed to keep waiting just to give a statement?

You don't have to wait but it can be very helpful, depending on the seriousness of the crime. Two men fighting and nobody dies, meh. A robbery or something more serious, could be worth waiting longer. You can always call dispatch and let them know you are heading home but provide your phone number. I've taken plenty of witness statements by phone or driven to their home to speak with them.