E.g for a runner with a 10k best pace of 04:00 min/km, would recover fastest (burning/clearing lactate) somewhere around the 06:00 min/km mark. Depends much on the runner's actual metabolic capacites (VO2max & VLamax) even if we know he do 10k in 40min - but running at that optimal recovery pace zone for around 10-15minutes after an all out 400m should clear lactate levels down to baseline (i.e muscles are no longer inhibited by acid for the next interval of anaerobic character). If he/she would be sitting/standing for recovery instead it will twice as long (or in that ballpark).
One would think "quick recovery - why not always?" But to your good question: There is acutally purpose for both passive & active rest (according to swim Dr/coach Jan Olbrecht). Passive rest in a high anaerobic interval set (e.g say between 100-400m, i.e duration of 10-60s) increases the anaerobic proportion of the whole workout. If the target is to increase anaerobic (glycolytic) capacity, this can be helpful to make sure one get actually desired adaptation in the energy systems for the very workout.
Active rest increases the aerobic proportion of the workout (you will be spending significant time doing long run-pace between intervals). I believe there is no situation where you wouldn't want active rest in intervals longer than 400m (more time efficient workouts).