Assume you are out on your morning ride to work. Suddenly you realize you forgot XX. How far is too far to go back home and get XX?
Let's assume that XX is of average importance, not critical to your job, but important. If it's something that's critical, the assumption is you'd go back for that no matter the distance.
...forgot my socks this morning, had to wear my white socks with my slacks, shirt and tie. Nice!
I had only gone one mile of 14. I didn't want to go back.