As with any workout (on the track or in the weight room), it depends on the intent and desired outcome, as well as the time of the year (point in your training progression).
As others have mentioned, your prescribed intensity and rest intervals will determine the effect of the training session.
Breakdowns can be as follows:
3 x 30 at 90% with 2 min recoveries
4 min set break
3 x 30 at 95% with 3 min recoveries
5 min set break
3 x 30 at 100% with 4-5 min recoveries
It may also depend on your start method (i.e. standing start, 3-point start, block start, push-up start), as more intense start positions will require greater recovery periods.
A higher intensity (i.e. higher velocity) workout would be as follows:
3 x 30m at 100% with 6 min recoveries
8 min set break
3 x 30m at 100% with 6-7 min recoveries
8-10 min set break
3 x 30m at 100% with 6-8 min recoveries
However, you may modify recovery periods if the stopwatch shows you are not maintaining quality (i.e. opening up the recoveries periods to maintain quality). Or, you may simply stop the workout if the times are getting slower over each rep (or you see a technical breakdown).
This is where someone of Charlie’s technical expertise and experience excels at coaching. Knowing when to increase the recovery period, changing the work or stopping the workout.
Alternatively, you can do 9 x 30 with a walk-back recovery at 80-85% - but it would not be considered a speed workout. This may be something you do with an athlete who is recovering from injury and you are trying to build up work capacity over short distances. At a lower intensity, it may be a tempo workout for a 300lb offensive lineman in football.