Yes, you can keep your DSLR camera on for two or three years based on the following assumptions:

• Your particular model DSLR includes an option that allows you to disable any 'sleep' option that is enabled by the factory default settings.
• Your memory card has sufficient capacity to hold that many photos taken at the 'Image Quality' you have selected. Obviously, lower resolution and higher compression will allow you to store more images on the same card. Assuming 10MB per image, it would take a little over 170GB per year. File sizes of 500KB per image would only require 8.5GB per year.
• The camera is capable of being programmed to take an image once every 30 minutes, either internally or via a remote timer. In the case of the remote timer, the power supply of the timer would need to be sufficient to last the entire term of the project.
• Alternately, if your DSLR is capable you could shoot with the camera tethered to a computer with enough free storage space to store the image data. An option to transfer each image to the computer from the camera's image processor rather than storing it on the camera's memory card would be required. The computer would need to be sufficiently stable to run uninterrupted for the entire term of your project.
• The operating environment is within the manufacturer's recommendation, preferably near room temperature and the exposure time for each frame is typical at less than 30 seconds. If the exposure time is longer or the operating environment falls outside manufacturer recommended parameters, then the likelihood of issues would significantly increase.
• The electrical circuit supplying the camera's power adapter with current is uninterrupted for the entire term of your project. In the case of tethering, the same would be required of the circuit supplying power to the computer.