The short answer
Fast enough that speed stops being your problem. The classic Harvard Business Review study of 2.24 million leads found that firms contacting a prospect within an hour were about seven times likelier to qualify that lead than those who waited even one more hour, and sixty times likelier than those who waited a day. So the routing rule is simple: assign instantly, notify in the channel the rep already lives in, and set a response SLA in minutes rather than hours. Keep the routing logic itself boring (territory, then segment, then round robin among the eligible reps) with an explicit fallback owner, because a clever routing tree with no default is how leads quietly rot in an unassigned queue.