Great question! And I have an answer….
It depends. I know, I say that a lot – but this is the thread you should be noticing. The answer so often does DEPEND on your goals, your methodology, your available time, your interests….
The ideal length of a workout is the length of a workout you can & will do that still allows you to achieve your goals. And the goals piece further solidifies it.
If you’re learning to run 10k there’s a certain amount of time that carries with it. The program I teach is 3 runs per week varying in length from 30-70+ minutes. But if your schedule doesn’t allow for that time commitment – perhaps training for a 5k is more appropriate. Still enables you to achieve the goal, but with a modification – one that sets you up for success.
When it comes to strength training, the answer will vary as well. If your goal is health and general strength & conditioning I think that 30 minutes 3 times per week is more than enough if you’re being efficient & effective with your time. BUT, if you’re training to be in the cross-fit games or to make a certain lift that number would potentially be different.
So, what is my bottom line answer? The MOST important factor? The amount of time you’ll actually DO without undue stress on your other life commitments. Because long term success requires consistency – no matter what your program says. So listen, be curious & create a routine that works for you!