On average, an earth auger can drill holes that are up to 3 feet deep. However, if you need to dig deeper holes, some of the choices come provided with special extension rods that you can successfully use to get the job done.
Once again, the depth of the holes that a device of this kind can dig depends on the specifications of each model in particular. So, before you make up your mind and decide to order a specific product, it is recommended that you do the needed research by looking at what the manufacturer has to say about the augers that it distributes.
If you want to be confident that the unit that you like can get the task done, it can also help to get in touch with previous buyers of the same model. This way, you can get a first-hand review of how a certain item performs in various circumstances.