Some drills are designed for heavy-duty use and will be able to run continuously for the longest periods of time without experiencing mechanical failure. Light-duty and medium-duty drills are more likely to stop working if they are used for jobs that require too much power or force the drill to run for an extended period of time -- more than the few minutes that most drilling jobs require. Drills with higher amp ratings and higher horsepower ratings are also more likely to run longer without problems.
A drill's usage will have a significant impact on how long it can run continuously. For example, drilling for extended periods into dense material such as steel or concrete will cause a drill to fail long before drilling into a soft material such as wood. Running a variable-speed drill at its highest setting will place the most strain on its motor, causing it to wear out faster than using it at a lower setting. Proper maintenance, storage and drill bit insertion will all prolong the life of a drill under constant use.
Cordless electric drills that run on batteries have another limiting factor when it comes to constant use: battery life. The amount of time a drill's battery will last depends on the battery's age, its charge capacity and the amount of power the drill is consuming on its current settings. Unless the drill is old or in poor condition, the battery will likely die before the motor fails.
Every drill, regardless of its rating and durability, will eventually break down if it's used improperly. Other than specialized industrial models, electric drills are not meant to be used continuously for hours or days at a time. When an electric drill experiences failure, it typically involves the bearing or the electric motor. A motor that burns out can create a fire hazard, which makes using a drill beyond its intended application dangerous. A bearing failure will simply cause the drill to stop spinning, which may also present a danger depending on the drill's use when it fails.