I usually make pulled chicken in the crock pots on both high and low for 4-8 hours. What I choose is dependent on if I'll be home, awake, etc. when it's done.
What is too long? Would 12 hours on low be to long for something like pulled chicken breasts?
You can safely leave a slow cooker on for 12 hours at low given there's enough liquid. Will it overcook the chicken? Almost certainly.
The only difference between low and high is the amount of time it takes to simmer. In other words, they will both reach 209 F but at different rates.