I go by what time I want to eat dinner. I don't work, so I never use the low settings as I would have to get up very early early to have it all in the pot to let it cook for 8 or 10 hours, and have it be done by 6pm or so. I also have a Rival with the high 4, 6 hour/ low 8, 10 hour settings, so I usually use the high 6 hours or 4 hours, depending on the recipe directions. My present crockpot does have a warm setting which it automatically switches to if I don't turn it off right after the 6 or 4 hours are up. So if you work a long day and use the low 10 hours, it will switch to warm after the 10 hours are up and keep it warm for you if you aren't home yet. None of the crock pots I used to own had a warm setting it was just low or high knob. I love having the warm setting and use it on holidays to make my mashed potatoes keep them at a perfect temp. till serving time, and I can concentrate on the rest of the meal.