From the Infinite Flight ATC Manual:
16.4.8 — When aircraft slow down, they may need to configure; Controllers should avoid slowing aircraft down where they will have to configure until within 20 track miles to touchdown – expeditious approaches are only possible if aircraft keep their speed up until close to the airport.
If you were wondering about the conversion from miles to nautical miles, 20 track miles is 17 nautical miles. It is referring flaps or retracting gear, for example, when it references that they “they may need to configure”.
Your goal as Center or Approach is to get the pilot off your frequency as quickly as you can. You don’t want to slow things down, you’re going to want them to get in as quick as possible.
For every new pilot on frequency, one needs leave. If you aren’t getting aircraft out at a decent rate you’ll begin to become overwhelmed.
Now the problem becomes, “when should I intervene” if you want to get them in faster. This becomes increasingly difficult decision to make in airspaces with higher traffic. You should set restrictions for yourself as to what speeds you will use and where you’ll apply them when necessary. I’ll slow them to 260 knots here, 230 knots here, 210 knots here and 180 knots here.
If there’s no conflict to avoid, leave them be. If there is a conflict you can see, intervene, speed them up.