If the event you are training for is more than 30 minutes, which is likely if you’re on these forums, you are better off doing fewer, longer workouts. Your best bet is carve out an hour in the mornings where you can run before you eat breakfast so you also get the fasted benefit.

4nthrax;

While Adamsc is correct that longer is better you gotta do what you can do first and figure out how it effects you and what you can handle. Certainly if you are time crunched then 2-3 x 30 min is way better than nothing. The minimum duration related to your fitness level. Do a thought experiment with me: Imagine your grandmother, who, for this experiment we are going to assume has not run a step since she was a teenager. For her a 30 minute run will have a profound training effect. Now imagine Kilian Jornet (if you can). For him a 30 minute run will not be enough training stimulus to add anything to his fitness. In fact too many days of 30 minute runs and he will LOSE fitness.

Can you see now that it is not so formulaic like you might have read on the internet. Training must be progressive to be effective. You must gradually increase the load of your body will stop adapting. Initially a 30 min might be a great starting point and really boost fitness for you but if you are still doing 1 x 30 min run 6 months from now there is a good chance that it will no longer be a sufficient training stimulus to cause you more adaptation. In that case, first you will plateau and soon drop in fitness because your body has become used to this training load.

I hope this helps. There is no one size fits all prescription.
Scott