Varies with the person and generally comes to a stop at some level of dis-comfort and in-security. Clothing layers and sleeping pads usually identify the Spartans :) Durability, expense, and volume are other limiting factors. I diverge from SUL with first aid and other essentials--- the in-security thing. The tent vs. tarp quandary is usually a security issue: bugs and creepy-crawlies. Toy separation anxiety can creep in too: "put down that MP3 player, punk!"
And there are some hidden factors that need analysis in a gear list: overnight or thru-hike? Two weeks in August or 3 season? Solo or group?
You will know by a factor of "too": if you are too wet, too cold, too tired from lack of sleep, too hungry, too dirty, too scared, or too lost, you might need other options (or training).