By the old school definition, it's supposed to be no more than 20 minutes. Back in my high school, we do tempo runs no more than 30 minutes. Tempo runs that I do are around 25 minutes.

I personally think that if a tempo run exceeds 40 minutes, I'm not in that consistent steady pace that will bring me to that anaerobic threshold...or if I did keep it up, the 40 minute long duration for a tempo run might seem to over stress my body. It seems like either an easy run or eventually trashing my body if I were to keep the tempo pace up for 40 minutes.

Lots of people nowadays do tempo runs for more than an hour! I'm very confused. Personally, it seems too much time for all that acidity to hang around in the body during the tempo run.

How should tempo runs be like? 20 minutes is definitely quick for others but over an hour?