You grind missions, gain recruits, and train because they are there. But if you play often enough, you gain more recruits faster than they are used by your training camps, so you end up with a backlog.
In other words, how many days could you stop playing the game altogether while continuing to produce heroes from your training camp?
I run one level 13 and three level 20 camps, and I am currently backlogged 40 days on all of them. It would be more if I hadn’t stopped running missions for a full week or if, much earlier in the game, I would max out my recruits and hold onto them until they were needed right before the previous training ended.
How about you?