Everyone is different so I guess it comes down to preference. I prefer to keep smoking as long as I am enjoying it. When I taste ash, I dump. Sometimes there is no dottle left but most of the time there is.
I would also suggest Savinelli but you could also look at Rossi. Rossi is made by Savinelli and they come in the same shapes and smoking quality. You could get two Rossi pipes for the price of one Savinelli.