This was referred to earlier when the auto-shutoff feature was discussed. The reason why this feature can be so useful is yes, you can incubate too long. Now, it is impossible to say if there is a set time that is too long because it depends on the temperature at which you are incubating. If you want to do so slowly at a lower temperature, then it can take anywhere from four to eight hours.
Heck, there have even been accounts where users ferment for 24 hours. However, if the yogurt is left inside too long then it can begin to curdle. We can’t speak for everyone, but most of you are probably not a fan of anything that curdles.